Display device and operation method thereof

ABSTRACT

To provide a display device having an input means using a bending action. A display device includes a first substrate, a second substrate, a display part, a first sensor element, and a second sensor element. The display part is provided between the first substrate and the second substrate. The first sensor element and the second sensor element are provided in their respective regions overlapping with the display part between the first substrate and the second substrate. The region where the second sensor element is provided overlaps with the region where the first sensor element is provided. The first substrate and the second substrate have flexibility. The first sensor element has a function of detecting the presence or absence of an object touching the first substrate or the second substrate. The second sensor element has a function of detecting the distortion of the first substrate or the second substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice and a display device including the semiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture, aprogram, or a composition of matter. Specifically, examples of thetechnical field of one embodiment of the present invention disclosed inthis specification include a semiconductor device, a display device, aliquid crystal display device, a light-emitting device, a lightingdevice, a power storage device, a storage device, a method for drivingany of them, and a method for manufacturing any of them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a storage device, adisplay device, or an electronic device includes a semiconductor device.

2. Description of the Related Art

In recent years, display devices have been applied to a variety of usesand essential to portable electronic appliances and the like. Displaydevices used for portable electronic appliances and the like arerequired to be thin and lightweight and to have a narrow bezel, forexample. Furthermore, display devices to which flexibility is impartedare expected to be applied to other uses.

For example, Patent Document 1 discloses, as a flexible display device,an active matrix light-emitting device in which an organic EL elementand a transistor serving as a switching element are provided over a filmsubstrate.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2003-174153

SUMMARY OF THE INVENTION

Application of a flexible display device utilizing its flexibility as ina curved display or the like is expected. Furthermore, an action such asbending can be used as an input means.

An object of one embodiment of the present invention is to provide aflexible display device. Another object is to provide a display devicehaving an input means using a bending action. Another object is toprovide a lightweight display device. Another object is to provide athin display device. Another object is to provide a display deviceincluding a distortion sensor. Another object is to provide a displaydevice including a touch sensor. Another object is to provide a noveldisplay device or the like. Another object is to provide an operationmethod of the display device. Another object is to provide a program foroperating the display device. Another object is to provide a novelsemiconductor device or the like. Another object is to provide anoperation method of the semiconductor device or the like. Another objectis to provide a program for operating the semiconductor device or thelike.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention relates to a display devicehaving an input means using the action of bending the display device.

One embodiment of the present invention is a display device including afirst substrate, a second substrate, a first element layer, and a secondelement layer. The first substrate and the second substrate haveflexibility. The first element layer is provided between the firstsubstrate and the second substrate. The second element layer is providedbetween the first substrate and the second substrate. The first elementlayer and the second element layer overlap with each other. The firstelement layer includes a display part and a first sensor element part. Atop surface shape of the display part is substantially rectangular. Thefirst sensor element part overlaps with the display part. The secondelement layer includes second sensor element parts. The second sensorelement parts overlap with a first region along a first side of thedisplay part, a second region along a second side facing the first side,and a third region intermediate between the first side and the secondside. The first sensor element part is configured to detect presence orabsence of an object touching the first substrate or the secondsubstrate. The second sensor element parts are configured to detectdistortion of the first substrate or the second substrate.

In this specification, ordinal numbers such as “first”, “second”, andthe like are used in order to avoid confusion among components, and theterms do not limit the components numerically.

Another embodiment of the present invention is a display deviceincluding a first substrate, a second substrate, a third substrate, afourth substrate, a first element layer, and a second element layer. Thefirst substrate and the second substrate have flexibility. The thirdsubstrate is provided between the first substrate and the secondsubstrate. The fourth substrate is provided between the first substrateand the second substrate. The third substrate and the fourth substrateare provided next to each other in positions not overlapping with eachother. The first element layer is provided between the first substrateand the second substrate. The first element layer has a regionoverlapping with the third substrate. The first element layer has aregion overlapping with the fourth substrate. The second element layeris provided between the first substrate and the second substrate. Thesecond element layer has a region overlapping with the third substrate.The second element layer has a region overlapping with the fourthsubstrate. The first element layer and the second element layer overlapwith each other. The first element layer includes a display part and afirst sensor element part. A top surface shape of the display part issubstantially rectangular. The first sensor element part overlaps withthe display part. The second element layer includes second sensorelement parts. The second sensor element parts overlap with a firstregion along a first side of the display part, a second region along asecond side facing the first side, and a third region intermediatebetween the first side and the second side. The second sensor elementparts overlap with the third substrate and/or the fourth substrate. Thefirst sensor element part is configured to detect presence or absence ofan object touching the first substrate or the second substrate. Thesecond sensor element parts are configured to detect distortion of thefirst substrate or the second substrate.

The display part is capable of being bent along a region between thethird substrate and the fourth substrate when seen from a displaysurface.

It is preferable that the third substrate be more rigid than the firstsubstrate and the second substrate and the fourth substrate be morerigid than the first substrate and the second substrate.

The third substrate and the fourth substrate have flexibility.

An insulating layer may be provided between the first element layer andthe second element layer.

The first sensor element can be a metal thin film resistor.

The display part can include an organic EL element.

Another embodiment of the present invention is a method for operating adisplay device, including: a first step of displaying a first image on adisplay part; a second step of determining whether a curvature of anedge or a vicinity of a center of the display part is larger than orequal to a first set value; a third step of displaying an image foroperation on the edge of the display part; a fourth step of detecting,on the image for operation, a position of an object touching the image;a fifth step of detecting, on the image for operation, a position of theobject touching the image; a sixth step of determining whether acurvature of the edge of the display part is larger than or equal to asecond set value; and a seventh step of changing the first image to asecond image. When the curvature is smaller than the first set value inthe second step, the determination of the curvature is performed again.When the curvature is larger than or equal to the first set value in thesecond step, a process proceeds to the third step. When a change in theposition of the object is not detected in the fifth step, the processproceeds to the sixth step. When a change in the position of the objectis detected in the fifth step, the process proceeds to the seventh step.When the curvature is smaller than the second set value in the sixthstep, the process returns to the fifth step. When the curvature islarger than or equal to the second set value in the sixth step, theprocess proceeds to the seventh step.

The process from the first step to the seventh step may be performedsequentially and repeatedly while the image displayed on the displaypart in the seventh step is used as the image displayed on the displaypart in the first step.

With one embodiment of the present invention, a flexible display devicecan be provided, a display device having an input means using a bendingaction can be provided, a lightweight display device can be provided, athin display device can be provided, a display device including adistortion sensor can be provided, a display device including a touchsensor can be provided, a novel display device or the like can beprovided, an operation method of the display device can be provided, aprogram for operating the display device can be provided, a novelsemiconductor device or the like can be provided, an operation method ofthe semiconductor device or the like can be provided, or a program foroperating the semiconductor device or the like can be provided

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating adisplay device.

FIGS. 2A to 2C are cross-sectional views each illustrating a displaydevice.

FIGS. 3A and 3B are a top view and a cross-sectional view illustrating adisplay device.

FIGS. 4A to 4F are top views illustrating the positions of regions wheredistortion sensor elements are provided.

FIG. 5 is a top view illustrating a display device.

FIGS. 6A and 6B are a cross-sectional view illustrating a display deviceand a top view of a distortion sensor element.

FIG. 7 is a cross-sectional view illustrating a display device.

FIG. 8 is a block diagram illustrating a display device.

FIGS. 9A and 9B illustrate modes of a display device.

FIGS. 10A to 10C illustrate modes of a display device.

FIGS. 11A and 11B illustrate operations of a display device.

FIGS. 12A and 12B illustrate operations of a display device.

FIGS. 13A and 13B illustrate operations of a display device.

FIGS. 14A and 14B each illustrate a distortion sensor circuit.

FIG. 15 illustrates a distortion sensor circuit and a pixel circuit.

FIG. 16 illustrates a distortion sensor circuit and a pixel circuit.

FIG. 17 illustrates a distortion sensor circuit and a pixel circuit.

FIG. 18 illustrates a distortion sensor circuit and a pixel circuit.

FIG. 19 illustrates a read circuit.

FIG. 20 illustrates a distortion sensor circuit and a pixel circuit.

FIG. 21 illustrates a distortion sensor circuit and a pixel circuit.

FIGS. 22A and 22B are each a cross-sectional view illustrating atransistor.

FIGS. 23A and 23B are each a cross-sectional view illustrating atransistor.

FIGS. 24A and 24B are each a cross-sectional view illustrating atransistor.

FIGS. 25A and 25B are each a cross-sectional view illustrating atransistor.

FIGS. 26A and 26B are each a cross-sectional view illustrating atransistor.

FIGS. 27A and 27B are each a cross-sectional view illustrating atransistor.

FIGS. 28A and 28B are each a cross-sectional view illustrating atransistor.

FIGS. 29A and 29B are each a cross-sectional view illustrating atransistor.

FIGS. 30A and 30B are each a top view illustrating a transistor.

FIG. 31 is a perspective view illustrating a display module.

FIGS. 32A and 32B illustrate an example of a touch panel.

FIGS. 33A, 33B1, 33B2, and 33C illustrate examples of configurations anddriving methods of a sensing circuit and a converter.

FIGS. 34A to 34C illustrate an example of a sensing circuit.

FIG. 35 is a cross-sectional view illustrating a touch panel.

FIGS. 36A and 36B illustrate an electronic appliance.

FIG. 37 is a flow chart of operation of a display device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to drawings. Notethat the present invention is not limited to the following descriptionand it will be readily appreciated by those skilled in the art thatmodes and details can be modified in various ways without departing fromthe spirit and the scope of the present invention. Therefore, thepresent invention should not be interpreted as being limited to thedescription of embodiments below. Note that in structures of the presentinvention described below, the same portions or portions having similarfunctions are denoted by the same reference numerals in differentdrawings, and description thereof is not repeated in some cases. It isalso to be noted that the same components are denoted by differenthatching patterns in different drawings, or the hatching patterns areomitted in some cases.

Note that in this specification and the like, when it is explicitlydescribed that X and Y are connected, the case where X and Y areelectrically connected, the case where X and Y are functionallyconnected, and the case where X and Y are directly connected areincluded therein. Here, X and Y each denote an object (e.g., a device,an element, a circuit, a wiring, an electrode, a terminal, a conductivefilm, or a layer). Accordingly, a connection relationship other thanthose shown in drawings and texts is also included without limitation toa predetermined connection relationship, for example, the connectionrelationship shown in the drawings and the texts.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. A switch is controlled to be on or off. Thatis, a switch is turned on or off (is brought into an on state or an offstate) to determine whether current flows therethrough or not.Alternatively, the switch has a function of selecting and changing acurrent path.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; amemory circuit; and/or a control circuit) can be connected between X andY. Note that for example, in the case where a signal outputted from X istransmitted to Y even when another circuit is interposed between X andY, X and Y are functionally connected.

Note that when it is explicitly described that X and Y are connected,the case where X and Y are electrically connected (i.e., the case whereX and Y are connected with another element or another circuit positionedtherebetween), the case where X and Y are functionally connected (i.e.,the case where X and Y are functionally connected with another elementor another circuit positioned therebetween), and the case where X and Yare directly connected (i.e., the case where X and Y are connectedwithout another element or another circuit positioned therebetween) areincluded therein. That is, when it is explicitly described that “X and Yare electrically connected”, the description is the same as the casewhere it is explicitly only described that “X and Y are connected”.

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

Note that for example, the case where a source (or a first terminal orthe like) of a transistor is electrically connected to X through (or notthrough) Z1 and a drain (or a second terminal or the like) of thetransistor is electrically connected to Y through (or not through) Z2,or the case where a source (or a first terminal or the like) of atransistor is directly connected to one part of Z1 and another part ofZ1 is directly connected to X while a drain (or a second terminal or thelike) of the transistor is directly connected to one part of Z2 andanother part of Z2 is directly connected to Y, can be expressed by usingany of the following expressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit configuration is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope. Note that these expressions are examples and there isno limitation on the expressions. Here, X, Y, Z1, and Z2 each denote anobject (e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer).

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. In addition, the term “insulating film” can be changed intothe term “insulating layer” in some cases.

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention will be described with reference to drawings.

The “display device” in this specification means an image display deviceor a light source (including a lighting device). Further, the displaydevice includes any of the following modules in its category: a moduleincluding a connector such as a flexible printed circuit (FPC), or tapecarrier package (TCP); a module including TCP which is provided with aprinted wiring board at the end thereof; and a module including a drivercircuit which is directly mounted on a display element by a chip onglass (COG) method.

The display device that is one embodiment of the present invention hasflexibility (flexible display device). Note that “flexible device” meansthat a device can be bent or warped.

FIG. 1A is a schematic top view illustrating a display device of oneembodiment of the present invention. FIG. 1B is a cross-sectional viewof FIG. 1A. The display device includes a substrate 41, a substrate 42,a substrate 43, a substrate 44, a substrate 48, an element layer 51, andan element layer 52. Note that the substrate 42 is not shown in FIG. 1A.

The element layer 51 is provided in contact with one surface of thesubstrate 48, and the substrate 42 is provided in contact with theelement layer 51. The element layer 52 is provided in contact with theother surface of the substrate 48, and the substrate 43 and thesubstrate 44 are provided in contact with the element layer 52. Here,the substrate 43 and the substrate 44 are provided with a spacetherebetween so as not to overlap with each other. The substrate 41 isprovided in contact with the element layer 52, the substrate 43, and thesubstrate 44.

The substrate 41, the substrate 42, and the substrate 48 haveflexibility. It is preferable that the substrate 41, the substrate 42,and the substrate 48 be formed using the same material and have the samethickness to prevent a warp due to thermal expansion or the like. Notethat the substrate 41, 42, or 48 and the layer or the substrate incontact therewith can be bonded to each other using a bonding layer orthe like not shown.

Note that the substrate 48 may be an insulating layer formed of siliconoxide, aluminum oxide, or the like. As shown in FIG. 2A, the substrate48 is not necessarily provided.

The substrate 43 and the substrate 44 are preferably more rigid than thesubstrate 41, the substrate 42, and the substrate 48. In the displaydevice having such a structure, a specific region can be bent easily. Inthe structure shown in FIGS. 1A and 1B, a region between the substrate43 and the substrate 44 can be bent easily. In addition, a region outerthan a side of the substrate 43 which is substantially parallel tofacing sides of the substrate 43 and the substrate 44 and is fartherfrom the substrate 44 in the top view can be bent easily. Furthermore, aregion outer than a side of the substrate 44 which is substantiallyparallel to facing sides of the substrate 43 and the substrate 44 and isfarther from the substrate 43 in the top view can be bent easily.

The rigidity of the substrate 43 and the substrate 44 may be the same asthat of the substrate 41, the substrate 42, and the substrate 48. Inthis structure, the mechanical strength of a region including thesubstrate 43 and the substrate 44 is increased owing to the thicknessesof the substrate 43 and the substrate 44. Therefore, in a manner similarto that described above, a specific region can be bent easily. In thecase where the thickness of the substrate 41 is partly increased, thesubstrate 43 and the substrate 44 may be omitted.

Note that the material, the thickness, and the like of each of thesubstrates may be selected as appropriate so that given regions can havethe same degree of flexibility. In this case, the substrate 43 and thesubstrate 44 may be omitted as shown in FIGS. 3A and 3B.

The element layer 51 includes a display part 47 and a touch sensor 49.As a display element provided in the display part 47, typically, anorganic EL element can be used. Instead of an organic EL element, aninorganic EL element can be used. Alternatively, a liquid crystalelement can be used. For example, a reflective display device can beprovided by combination of a liquid crystal element and a reflectiveelectrode. A transmissive display device can be provided by combinationof a liquid crystal element and a thin light source such as an organicEL element. A semi-transmissive display device can be provided bycombination of a liquid crystal element, a reflective electrode, and athin light source.

As the touch sensor 49, a capacitive-type touch sensor, an optical-typetouch sensor, a pressure-sensitive type touch sensor, or the like can beused. Note that the position of the touch sensor 49 is not limited tothat overlapping with the display part 47 as shown in the drawing. Thetouch sensor 49 may be incorporated in the display part. Alternatively,the touch sensor 49 may be provided between the substrate 42 and theelement layer 51. Still alternatively, the touch sensor 49 may beprovided over the substrate 42.

The element layer 51 includes a first circuit electrically connected tothe display element. The first circuit can have a function as a pixelcircuit. When the display element is an organic EL element, a circuitconfiguration including two transistors and one capacitor can beemployed, for example. When the display element is a liquid crystalelement, a circuit configuration including one transistor and onecapacitor can be employed.

The element layer 52 includes a sensor capable of indirectly sensingdistortion of the substrate 41 and/or the substrate 42. Specifically, adistortion sensor element can be used as the sensor. As the distortionsensor element, typically, a metal thin film resistor can be used. Theamount of distortion in the vicinity of the region where the metal thinfilm resistor is provided can be measured on the basis of the amount ofchange in the resistance of the metal thin film resistor. As thedistortion sensor element, a piezoelectric element can also be used. Asthe piezoelectric element, an element including a piezoelectricsubstance such as barium titanate, lead zirconate titanate, or zincoxide can be used.

The distortion sensor element is provided in a region 46 shown in FIGS.1A and 1B, for example. Note that the number of distortion sensorelements to be provided in the region 46 is not limited to one, and aplurality of distortion sensor elements may be provided in the region46. Although the region 46 partly overlaps with the substrate 43 and/orthe substrate 44 in FIGS. 1A and 1B, a structure in which the region 46overlaps with neither the substrate 43 nor the substrate 44 can beemployed, or a structure in which the region 46 wholly overlaps with thesubstrate 43 and/or the substrate 44 can be employed.

The element layer 52 can include a second circuit electrically connectedto the distortion sensor element. The second circuit has a function ofreading the amount of change in the resistance of the distortion sensorelement. The second circuit can include, for example, two or threetransistors and a resistor. Note that the second circuit is notnecessarily included in the element layer 52 and may be attachedexternally.

Alternatively, as shown in FIG. 2B, the display part 47 and the region46 including the distortion sensor element may be provided in theelement layer 51. In this case, the element layer 51 can include thefirst circuit electrically connected to the display element and thesecond circuit electrically connected to the distortion sensor element.Note that the second circuit may be attached externally.

Still alternatively, as shown in FIG. 2C, a substrate 45 may be providedbetween the element layer 52 and the substrate 43, the substrate 44, andthe substrate 41. Note that the substrate 45 may be an insulating layer.

The orientation of the distortion sensor element provided in the region46 is set so that the distortion sensor element can detect a bend in adirection indicated by an arrow in the region 46 shown in FIG. 1A. Forexample, in the case where the top surface of the display part 47 issubstantially rectangular as shown in the drawing, the distortion sensorelements are provided in a region along a first side of the display part47, a region along a second side facing the first side, and a regionintermediate between the first side and the second side. Thus, theaction of bending the display device in half along its center and theaction of bending an edge of the display device can be sensed.

Linking the bending action to a display function enables the displaydevice to be operated by an action similar to that of reading a book.Specifically, operation such as displaying an image, turning off thedisplay, or changing the image can be performed by the action of openingor closing a book, turning a page, or the like given to the displaydevice.

Note that the region 46 where the distortion sensor element is providedis not limited to the positions in the examples shown in FIG. 1A andFIG. 3A, and may be provided in positions shown in FIGS. 4A to 4F. Evenwhen the region 46 is provided in the positions shown in FIGS. 4A to 4F,the above-described bending action can be detected. Note that the numberof and the distance between the regions 46 shown in FIG. 1A, FIG. 3A,and FIGS. 4A to 4F are examples and not limited to those shown in thedrawings.

FIG. 5 shows an example of a top view of the display part 47, thedistortion sensor element included in the region 46, and drivercircuits.

In the display part 47, pixels 81 are arranged in a matrix and eachinclude the display element and the first circuit. The first circuit iselectrically connected to a circuit 71 and a circuit 72. The circuit 71can function as a signal line driver circuit (source driver), forexample. The circuit 72 can function as a scan line driver circuit (gatedriver), for example.

A distortion sensor element 82 can be provided in part of the pixels 81or provided to overlap with part of the pixels 81.

The second circuit can be included in the pixel 81. Alternatively, thesecond circuit can be provided to overlap with the pixel 81. Stillalternatively, the second circuit can be attached externally. A circuit73 and a circuit 74 are a circuit selecting the distortion sensorelement 82 and a circuit for reading a signal, respectively. The secondcircuit can also be included in any of the circuit 73 and the circuit74.

FIG. 6A is an example of a cross-sectional view of part of a displaydevice that includes an organic EL element as a display element. Notethat FIG. 6A illustrates part of typical structures in a region 302including the display element in the pixel 81 and the distortion sensorelement 82 shown in FIG. 5, a region 306 including a driver circuit, anda flexible printed circuit (FPC) connection region 305. FIG. 6B is a topview of the distortion sensor element 82, and FIG. 6A illustrates across section along the dotted line in FIG. 6B. The distortion sensorelement 82 can detect a change in shape in the direction indicated bythe arrow. Note that detailed description of the touch sensor 49 isomitted.

The region 306 can include the second circuit, the circuit 71, thecircuit 72, the circuit 73, the circuit 74, or the like. Note that partor all of the second circuit, the circuit 71, the circuit 72, thecircuit 73, and the circuit 74 may be an external IC chip or external ICchips.

The display device illustrated in FIG. 6A, which is an example of thedisplay device illustrated in FIG. 1A, has a region in which thesubstrate 41, the substrate 43, the element layer 52, the substrate 48,an insulating film 321 a, the element layer 51, an insulating film 321b, and the substrate 42 are stacked in this order. Note that thesubstrate 41, the substrate 42, the substrate 43, or the substrate 48and the layer or the substrate in contact therewith may be provided witha bonding layer or the like (not shown) therebetween.

As the insulating film 321 a and the insulating film 321 b, a singlelayer of a silicon oxide film, a silicon oxynitride film, a siliconnitride film, or a silicon nitride oxide film, or a stacked layerincluding any of the films can be used.

In FIG. 6A, the element layer 51 includes a transistor 350, a transistor354 a, a transistor 354 b, an insulating film 364, an insulating film368, a planarization insulating film 370, a connection electrode 360, aconductive film 372, a conductive film 374, an insulating film 334, asealing layer 432, a coloring layer (color filter) 336, and alight-blocking layer (black matrix) 338. The element layer 51 ishermetically sealed with the substrate 41, the substrate 48, the sealinglayer 432, and a sealing material 312. Note that there is a case wherepart of the above components is not included or a component other thanthe above components is included in the element layer 51.

The element layer 52 includes the distortion sensor element 82 and aplanarization insulating film 371. Note that the second circuit forreading a change in the resistance of the distortion sensor element 82may be provided in the element layer 52.

The insulating film 364 can be formed using, for example, silicon oxideor silicon oxynitride. The insulating film 364 is preferably formedusing an oxide insulating film containing oxygen in excess of that inthe stoichiometric composition. The insulating film 368 has a functionof blocking oxygen, hydrogen, water, an alkali metal, an alkaline earthmetal, or the like. For example, a nitride insulating film is preferablyused.

The planarization insulating films 370 and 371 can be formed using aheat-resistant organic material, such as a polyimide resin, an acrylicresin, a polyimide amide resin, a benzocyclobutene resin, a polyamideresin, or an epoxy resin. Note that the planarization insulating films370 and 371 may be formed by stacking a plurality of insulating filmsformed from these materials.

In the region 302, the transistor 350 is included in the first circuitand is electrically connected to an organic EL element 480. The organicEL element 480 includes the conductive film 372, an EL layer 446, andthe conductive film 374. The display device illustrated in FIG. 6A iscapable of displaying an image by light emission from the EL layer 446included in the organic EL element 480.

An insulating film 430 is provided over the conductive film 372 over theplanarization insulating film 370. The insulating film 430 covers partof the conductive film 372. A conductive film with high properties ofreflecting light emitted from the EL layer 446 is used for theconductive film 372, and a conductive film with high properties oftransmitting the light is used for the conductive film 374, whereby theorganic EL element 480 can have a top emission structure. Alternatively,a conductive film with high properties of transmitting the light is usedfor the conductive film 372, and a conductive film with high propertiesof reflecting the light is used for the conductive film 374, whereby theorganic EL element 480 can have a bottom emission structure. Furtheralternatively, a conductive film with high properties of transmittingthe light is used for both the conductive film 372 and the conductivefilm 374, whereby a dual emission structure can be obtained.

For the insulating film 430, an organic resin or an inorganic insulatingmaterial can be used, for example. As the organic resin, for example, apolyimide resin, a polyamide resin, an acrylic resin, a siloxane resin,an epoxy resin, a phenol resin, or the like can be used. As theinorganic insulating material, silicon oxide, silicon oxynitride, or thelike can be used, for example.

The coloring layer 336 is provided to overlap with the organic ELelement 480, and the light-blocking layer 338 is provided to overlapwith the insulating film 430. The coloring layer 336 and thelight-blocking layer 338 are covered with the insulating film 334. Aspace between the organic EL element 480 and the insulating film 334 isfilled with the sealing layer 432.

For the sealing layer 432, a solid sealing material with flexibility canbe used. For example, a glass material such as a glass frit, or a resinmaterial such as a two-component-mixture-type resin which is curable atroom temperature, a light curable resin, a thermosetting resin, and thelike can be used.

Although a structure with the coloring layer 336 is described as thedisplay device in FIG. 6A, the structure is not limited thereto. In thecase where the EL layers 446 with different emission colors areselectively formed, the coloring layer 336 is not necessarily provided.The color of the coloring layer 336 is not limited to three colors of R(red), G (green), and B (blue). For example, a display unit may becomposed of four pixels of the R pixel, the G pixel, the B pixel, and aW (white) pixel. Alternatively, a color element may be composed of twocolors among R, G, and B as in PenTile layout. The two colors may differamong display units. Alternatively, one or more colors of yellow, cyan,magenta, and the like may be added to RGB. Further, the size of adisplay region may be different depending on respective dots of thecolor components. Embodiments of the disclosed invention are not limitedto a display device for color display; the disclosed invention can alsobe applied to a display device for monochrome display.

Each of the substrate 41, the substrate 42, the substrate 43, thesubstrate 44, and the substrate 48 is preferably formed using a materialwith high toughness. Thus, a light-emitting device with high impactresistance that is less likely to be broken can be provided. Forexample, when an organic resin substrate is used as the substrate 41 andthe substrate 42, the display device can be lightweight and unlikely tobroken as compared to the case where a glass substrate is used as thesubstrate.

For the substrate 41, the substrate 42, the substrate 43, the substrate44, and the substrate 48, for example, a material selected from thefollowing can be used: glass which is thin enough to have flexibility,polyester resins such as polyethylene terephthalate (PET) andpolyethylene naphthalate (PEN), a polyacrylonitrile resin, a polyimideresin, a polymethyl methacrylate resin, a polycarbonate (PC) resin, apolyethersulfone (PES) resin, a polyamide resin, a cycloolefin resin, apolystyrene resin, a polyamide imide resin, a polyvinyl chloride resin,and a polyether etherketone (PEEK) resin. In particular, a materialwhose thermal expansion coefficient is low is preferred, and forexample, a polyamide imide resin, a polyimide resin, or PET ispreferably used. A substrate in which a glass fiber is impregnated withan organic resin or a substrate whose thermal expansion coefficient isreduced by mixing an organic resin with an inorganic filler can also beused.

The FPC connection region 305 includes the connection electrode 360, ananisotropic conductive film 380, and an FPC 316. The connectionelectrode 360 can be formed in a step of forming the source electrodelayer and the drain electrode layer of the transistor. The connectionelectrode 360 is electrically connected to a terminal included in theFPC 316 through the anisotropic conductive film 380.

Note that the distortion sensor element 82 senses change in resistancedue to expansion and contraction of the metal thin film; thus, onesurface and the other surface of the metal thin film are preferably incontact with different materials having the respective mechanicalproperties (e.g., elastic modulus, bending strength, a Young's modulus,Poisson's ratio, or hardness) so that the metal thin film can easilyexpand and contract. For example, in the structure in FIG. 6A, onesurface is in contact with the planarization insulating film 371, andthe other surface is in contact with the substrate 41. Here, since theplanarization insulating film 371 and the substrate 41 are formed usingmaterials with different mechanical properties, the surface on which themetal thin film can expand and contract relatively easily is determinedTherefore, for example, the metal thin film expands when the shape ischanged to be convex, and the metal thin film contracts when the shapeis changed to be concave. Accordingly, the direction of change in shapecan be sensed on the basis of the resistance when the metal thin filmexpands and the resistance when the metal thin film contracts.

The display device of one embodiment of the present invention may have astructure illustrated in a cross-sectional view in FIG. 7. A displaydevice shown in FIG. 7 is an example of the display device shown in FIG.2B. Note that description of a portion common to the display deviceshown in FIGS. 6A and 6B is omitted.

FIG. 7 illustrates part of typical structures in the region 302including the display element in the pixel 81 and the distortion sensorelement 82 shown in FIG. 5, the region 306 including the driver circuit,and the flexible printed circuit (FPC) connection region 305.

In the region 306, the transistor 352 can be included in the secondcircuit. In this case, electrodes of the distortion sensor element 82are electrically connected to the respective wirings. One of the wiringsis electrically connected to a gate electrode of the transistor 352.

In the region 306, a transistor 354 can be included in the circuit 71,the circuit 72, the circuit 73, or the circuit 74.

In the structures of the display devices illustrated in FIG. 6A and FIG.7, two or more distortion sensor elements may be provided at differentheights (in the thickness direction) between the substrate 41 and thesubstrate 42. The distortion sensor element may be provided over thesubstrate 41, the substrate 42, or both the substrate 41 and thesubstrate 42.

The transistor in the above display device is preferably a transistorwhose channel formation region is formed in an oxide semiconductorlayer.

Since the transistor using an oxide semiconductor layer has highmobility, an area occupied by transistors can be made small, and theaperture ratio can be increased. With use of the transistor, thecircuits 71 to 74 can be formed over the substrate provided with thepixel 81. In addition, the transistor has extremely low off-statecurrent and can hold a video signal or the like for a longer period;thus, the frame frequency can be lowered, and the power consumption ofthe display device can be reduced.

Furthermore, a transistor including an oxide semiconductor layer ispreferably used as the transistor electrically connected to thedistortion sensor element 82. By the use of a transistor with anextremely low off-state current as the transistor, electric charge whichis unnecessarily input and output into and from an output wiring or thelike can be inhibited.

The oxide semiconductor layer preferably includes a c-axis alignedcrystal. In the case where the oxide semiconductor layer including thecrystal is used for a channel formation region of the transistor, acrack or the like is less likely to occur in the oxide semiconductorlayer when the display device is bent, for example. As a result, thereliability can be improved.

Note that the transistor including an oxide semiconductor layer ismerely an example, and a transistor including an amorphous silicon layeror a polycrystalline silicon layer can also be used. Alternatively, atransistor including an organic semiconductor may be used. Examples ofan organic semiconductor include acenes such as tetracene and pentacene,oligothiophene derivatives, phthalocyanines, perylene derivatives,rubrene, Alq3, TTF-TCNQ, polythiophene (e.g., poly(3-hexylthiophene)(P3HT)), polyacetylene, polyfluorene, polyphenylene vinylene,polypyrrole, polyaniline, anthracene, tetracyanoquinodimethane (TCNQ),and polyparaphenylene vinylene (PPV).

FIG. 8 is an example of the block diagram of a display device of oneembodiment of the present invention. A display device 5000 includes adisplay part 5001, a distortion sensor part 5002, a touch sensor part5009, a circuit 5010, a circuit 5020, a circuit 5030, a first memorydevice 5040, and a second memory device 5050. Note that a structure maybe employed in which components other than the display part 5001, thedistortion sensor part 5002, and the touch sensor part are not providedin the display device 5000 and externally provided. To the displaydevice 5000, various circuits such as a control circuit, an arithmeticcircuit, and a power supply circuit that are not shown in the drawing,and a memory circuit that is different from that in the above can beconnected.

Here, the circuit 5010 can have a function of controlling the displaypart 5001. The circuit 5020 can have a function of performing processingregarding information to be displayed on the display part 5001. Thecircuit 5030 can have a function of controlling the distortion sensorpart 5002 and the touch sensor part 5009. Note that circuits forcontrolling the distortion sensor part 5002 and the touch sensor part5009 may be provided. The first memory device 5040 and the second memorydevice 5050 can each have a function of storing data on information tobe displayed on the display part 5001 (the data is hereinafter referredto as display data). The first memory device 5040 may be a reproducingdevice which reads the display data from a memory medium.

First, a signal detected by the distortion sensor part 5002 and/or thetouch sensor part 5009 in the display part 5001 having flexibility isinput to the circuit 5020 through the circuit 5030.

The circuit 5020 reads necessary display data from the first memorydevice 5040 depending on the signal. Part of the display data can bestored in the second memory device 5050 and read from the second memorydevice 5050 as needed. Furthermore, data input by a user can be storedin the second memory device 5050. The supply of power to the displaypart 5001 can be controlled depending on the signal detected by thedistortion sensor part 5002.

The circuit 5020 can be connected to a communication device 5060. Thus,display data can be read from the outside through the communicationdevice 5060, and display data or data input by a user can be output tothe outside through the communication device 5060.

Display data read by the circuit 5020 is input to the circuit 5010, andan image based on the display data is displayed on the display part5001.

The repetition of the above-described operation can achieve linkagebetween a display function and the action of bending and/or touching thedisplay device, so that the display device can be operated by an actionsimilar to that of reading a book.

Application modes of the display device of one embodiment of the presentinvention are described. FIG. 9A shows an example of the display device5000 of one embodiment of the present invention which is opened to havea substantially flat surface. The display part 5001 can display, forexample, time 5101, an icon 5102 linked to the start of software, textinformation 5103, and an image 5104. By using a touch sensor included inthe display part 5001, the icon 5102 can be operated and screenswitching can be performed, for example. That is, the display device ofone embodiment of the present invention can be used as an informationterminal.

Next, one mode of the display device 5000 which is used as an electronicbook is described. In a state where the display device 5000 is opened tohave a flat surface like the state shown in FIG. 9A, informationselected by a user can be displayed on the entire display part 5001 asshown in FIG. 9B, for example.

Next, operation when the display device 5000 is bent is described.First, when the display device 5000 is bent along the vicinity of thecenter, an image 5003 of edges of sheets of paper bound together isdisplayed on each of two opposite edges of the display part as shown inFIG. 10A. Provided that the upper drawing in FIG. 10A is a front view,the lower drawing in FIG. 10A is a top view. A bend angle of the displaypart 5001 on which the image 5003 is displayed may be set arbitrarily.For example, the angle can be set to approximately 100° to 175°.

The image 5003 can be displayed also by bending the edge of the displaydevice 5000 as shown in FIG. 10B. Provided that the upper drawing inFIG. 10B is a front view, the lower drawing in FIG. 10B is a top view.

As shown in FIG. 10C, the widths of the images 5003 displayed on theright and the left sides of the display part 5001 can be made differentfrom each other so that a user can visually recognize the rough positionof a displayed page. For example, in the case where a book whose pagesare turned from left to right is assumed to be read using the displaydevice 5000, at the beginning of reading, the image 5003 having a smallwidth is displayed on the left side of the display part 5001 while theimage 5003 having a large width is displayed on the right side thereof.The widths of the images 5003 on the right and the left sides can bereversed at the end of the reading.

As shown in FIG. 11A, the image 5003 can include an index 5004.Furthermore, the edge of the display device 5000 can be bent so that thedisplay surface of the display part 5001 is convex curved as shown inFIG. 11B. In that case, the bent edge can display the image 5003 ofedges of sheets of paper bound together in which the distance betweenthe edges of the sheets of paper is larger as they are closer to theoutside of the display part. The width of the image 5003 can be changeddepending on the curvature of the edge of the display device 5000. Forexample, the larger the curvature is, the more the width of the image5003 is increased, as in the case of a book. In the case where thecurvature is small, the width of the image 5003 is decreased. In thecase where the curvature is small and a bent region of a center of thedisplay device 5000 has a small curvature, the width of the image 5003is further decreased to be closer to the display shown in FIG. 9B.

The above-described mode of the display device 5000 shown in FIGS. 10Ato 10C and FIGS. 11A and 11B resembles the shape and the visual form ofa real book.

Next, an example of a method for operating the display device 5000 isdescribed.

For example, a method for changing a page displayed on the display part5001 is described. First, the image 5003 is displayed by any of theabove-described methods. Then, a region where the image 5003 isdisplayed is touched with a finger or the like as shown in FIG. 12A andthe finger or the like is slid, whereby a given page can be displayed.For example, images can be changed so that one page is displayed at atime by slowly sliding the finger or the like. A target page can bepromptly displayed by quickly sliding the finger or the like. In thecase where the index 5004 is displayed, a target page can be displayedquickly by touching a region where the index 5004 is displayed. Theoperation is performed using functions of the distortion sensor part5002 and the touch sensor part 5009 in the display device 5000.

Alternatively, pages can be changed by bending the display device 5000so that the display surface of the display part 5001 is convex curved asshown in FIG. 12B. For example, it is possible to change one page everytime the action of bending and returning from the bent state isperformed. Furthermore, images can be changed successively with thedisplay device 5000 bent. The operation is performed using a function ofthe distortion sensor part 5002 in the display device 5000.

Note that when the image is changed, an image may be displayed whichlooks as if a page of a book were turned, or the image may be changedimmediately. In the case of performing the above-described operationusing a region where the image 5003 is displayed on the left side of thedisplay part 5001, the image can be changed as if a user turned a pagefrom left to right. In the case of performing the above-describedoperation using a region where the image 5003 is displayed on the rightside of the display part 5001, the image can be changed as if a userturned a page from right to left.

In the case where the image is changed as if a page of a book wereturned, the speed of changing the image can be adjusted using a bendangle of the display surface. For example, the image can be slowlychanged when the display surface is bent at a relatively large bendangle (closer to a flat state) as shown in FIG. 13A, whereas the imagecan be promptly changed when the display surface is bent at a relativelysmall bend angle as shown in FIG. 13B. The operation is performed usingthe function of the distortion sensor part 5002 in the display device5000.

The operation of the mode of the display device 5000 which is used as anelectronic book can be described with reference to a flow chart shown inFIG. 37.

First, an image is displayed on the display part 5001 (S101). The imageis selected by a user and shows a front cover of a book or a given pageof a book, for example.

When a user intends to change a page, the user bends the vicinity of thecenter or the edge of the display part 5001.

The display device 5000 determines, using a distortion sensor, whether acurvature formed by the bending action is larger than or equal to a setvalue (S102).

In the case where the curvature is smaller than the set value, theabove-described determination of the curvature is performed repeatedlywhile the image displayed in S101 is maintained.

In the case where the curvature is larger than or equal to the setvalue, the image 5003 for operation is displayed on the edge of thedisplay part (S103).

Next, when a finger of the user is put on the image 5003, for example,the position of the finger is detected by the touch sensor of thedisplay device 5000 (S104).

The user moves an object such as the finger on the image 5003, or bendsthe edge of the display part 5001.

The position of the finger is detected again by the touch sensor of thedisplay device 5000 using a timer or the like (S105). When the positionof the finger is changed at this time, a target image is selected andthe image is displayed (S107). That is, the operation of changing a pageis performed.

When a change in the position of the finger is not recognized in S105,the distortion sensor determines whether the curvature is larger than orequal to a set value (S106). In the case where the curvature is largerthan or equal to the set value at this time, a target image is selectedand the image is displayed (S107).

In the case where the curvature is smaller than the set value in S106,the image displayed in S101 is maintained, and the process returns toS105.

After the image is changed in S107, the process returns to S101. Theabove-described steps are performed repeatedly while the image displayedin S107 is used as the image displayed in S101.

Note that the flow chart shown in FIG. 37 is just an example; thedisplay device can be operated while part of the steps included in theflow chart is omitted. Alternatively, a step which is not included inthe flow chart may be added.

The display device 5000 may have a program in which the steps in theabove-mentioned flow chart are written. Alternatively, a control devicethat controls the display device 5000 may have the program. Furtheralternatively, the program may be stored in a memory medium.

Note that the region where the image 5003 is displayed and a function ofthe region may be separated from the display part 5001.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 2

In this embodiment, the distortion sensor element and the second circuithaving a function of reading the amount of change in resistance of thedistortion sensor element which are described in Embodiment 1, will bedescribed in more detail. Note that in this embodiment, the secondcircuit electrically connected to the distortion sensor element iscalled a distortion sensor circuit.

FIG. 14A illustrates an example of a distortion sensor circuit thatincludes, as a distortion sensor element, a variable resistor such as ametal thin film resistor. The distortion sensor circuit includes atransistor 501, a transistor 502, a variable resistor 510, and aresistor 520.

One of a source and a drain of the transistor 501 is electricallyconnected to a wiring 540, and the other of the source and the drain iselectrically connected to one of a source and a drain of the transistor502. The other of the source and the drain of the transistor 502 iselectrically connected to a wiring 550, and a gate thereof iselectrically connected to a wiring 530. One terminal of the variableresistor 510 is electrically connected to a wiring 560, and the otherterminal thereof is electrically connected to one terminal of theresistor 520 and a gate of the transistor 501. The other terminal of theresistor 520 is electrically connected to a wiring 570.

Here, the wiring 530 can function as a selection signal line of thedistortion sensor circuit. The wiring 540 can function as a high powersupply potential line. The wiring 550 can function as an output signalline. The wiring 560 can function as a high power supply potential line.The wiring 570 can function as a low power supply potential line. Notethat the wiring 560 may be a low power supply potential line and thewiring 570 may be a high power supply potential line.

Conduction and non-conduction of the transistor 501 are controlled withthe potential of a node ND1, and conduction and non-conduction of thetransistor 502 are controlled with the potential of the wiring 530.

For example, with the assumption that a potential Vpc2 of the wiring 540and the initial potential Vini of the wiring 550 satisfy Vini<Vpc2, whenthe potential of the wiring 530 is at the “H” level and the transistor502 is on, the potential Vout of the wiring 550 depends on whether thetransistor 501 is on or off. In other words, the potential Vout of thewiring 550 depends on the potential V1 of the node ND1, which is thegate potential of the transistor 501.

The potential of the node ND1 depends on the resistance R1 of thevariable resistor 510 and the resistance R2 of the resistor 520. Inother words, the potential V1 of the node ND1 can be expressed by theformula below where Vex1 represents the potential of the wiring 560 andVex2 represents the potential of the wiring 570.

$\begin{matrix}{{V\; 1} = {{\frac{R\; 2}{{R\; 1} + {R\; 2}}{Vex}\; 1} + {\frac{R\; 1}{{R\; 1} + {R\; 2}}{Vex}\; 2}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

According to the above formula, V1 decreases when the resistance R1 ofthe variable resistor 510 changes in the positive direction, and V1increases when the resistance R1 of the variable resistor 510 changes inthe negative direction. In the distortion sensor circuit, the potentialcorresponding to V1 is outputted to the wiring 550. Thus, the amount ofdistortion of the variable resistor 510 can be known from the potentialoutputted to the wiring 550.

Note that the distortion sensor circuit of one embodiment of the presentinvention may have a structure illustrated in FIG. 14B. The distortionsensor circuit in FIG. 14B is different from the distortion sensorcircuit in FIG. 14A in that it includes a transistor 503. One of asource and a drain of the transistor 503 is electrically connected toone terminal of the variable resistor 510, and the other of the sourceand the drain is electrically connected to the wiring 560. A gate of thetransistor 503 is electrically connected to the wiring 530.

The transistor 503 is on only when the potential of the wiring 530 is atthe “H” level, i.e., when the distortion sensor circuit is selected.Therefore, the transistor 503 is off when the potential of the wiring530 is at the “L” level, whereby current unnecessarily flowing betweenthe wiring 560 and the wiring 570 can be suppressed.

FIG. 15 illustrates an example of a circuit in which a distortion sensorcircuit part 91, which is similar to the circuit in FIG. 14A, and apixel circuit part 92 including an organic EL element are combined. Thepixel circuit part 92 includes a transistor 504, a transistor 505, acapacitor 610, and an organic EL element 620.

One of a source and a drain of the transistor 504 is electricallyconnected to a wiring 580, and the other of the source and the drain iselectrically connected to a gate of the transistor 505 and one terminalof the capacitor 610. A gate of the transistor 504 is electricallyconnected to a wiring 590. The other terminal of the capacitor 610 andone of a source and a drain of the transistor 505 are electricallyconnected to the wiring 560. The other of the source and the drain ofthe transistor 505 is electrically connected to one terminal of theorganic EL element 620, and the other terminal of the organic EL element620 is electrically connected to the wiring 570.

In the circuit illustrated in FIG. 15, the other terminal of the organicEL element 620 and the other terminal of the resistor 520 can beelectrically connected to each other.

Here, the wiring 580 can function as a signal line inputting an imagesignal to the pixel circuit part 92. The wiring 590 can function as aselection signal line of the pixel circuit part 92.

The transistor 504 is turned on when the potential of the wiring 590 isset at the “H” level, whereby data corresponding to the potential of thewiring 580 can be written into the node ND2. When the potential of thewiring 590 is set at the “L” level, the data in the node ND2 can bestored.

Conduction and non-conduction of the transistor 505 is controlled withthe potential of the node ND2. Accordingly, whether the organic ELelement 620 emits light or not can be controlled with the potential ofthe node ND2.

Note that the circuit may have a structure as illustrated in FIG. 16where the distortion sensor circuit part 91, which is similar to thecircuit in FIG. 14B, and the pixel circuit part 92 including the organicEL element are combined. The other terminal of the capacitor 610, one ofthe source and the drain of the transistor 505, and the other of thesource and the drain of the transistor 503 can be electrically connectedto one another.

Although one pixel circuit part 92 and one distortion sensor circuitpart 91 are combined in the circuits illustrated in FIGS. 15 and 16, theplurality of pixel circuit part 92 and one distortion sensor circuitpart 91 can be combined as illustrated in FIG. 17. Note that althoughfour pixel circuit part (pixel circuit part 92 a to 92 d) are shown inFIG. 17, one embodiment of the present invention is not limited to thisstructure and two or more pixel circuit parts and one distortion sensorcircuit part 91 can be combined. Note that as illustrated in FIG. 18,the distortion sensor circuit part 91 may include the transistor 503.

Note that the wiring 550 can be provided with a circuit having a readingfunction as illustrated in FIG. 19. The wiring 550 is connected to aselector 630, and a selected signal is inputted to a gate of atransistor 640. Here, a transistor 650 is a switch for resetting thegate potential of the transistor 640 to a potential supplied from awiring 545. A signal outputted from the transistor 640 can be outputtedas digital data through an amplifier 660 and an analog-digital converter670. The wiring 545 can function as a low power supply potential line.Note that the wiring 540 may be a low power supply potential line andthe wiring 545 may be a high power supply potential line when thecircuit having a reading function as illustrated in FIG. 19 is providedin circuits as illustrated in FIGS. 14 to 18.

Although the pixel circuit part 92 includes the organic EL element 620in the above examples, the pixel circuit part 92 may include a liquidcrystal element 625 as illustrated in FIGS. 20 and 21. It is needless tosay that the transistor 503 may be included in the circuits illustratedin FIGS. 20 and 21 as in the above-described circuits.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 3

In this embodiment, a transistor that can be used for a display deviceof one embodiment of the present invention and a material included inthe transistor will be described. The transistor described in thisembodiment can be used for the transistors 350, 352, 354, 501, 502, 503,504, 505, and the like described in the above embodiment.

FIG. 22A is a cross-sectional view of an example of a transistor thatcan be used in a display device of one embodiment of the presentinvention. The transistor includes an insulating film 915 over asubstrate 900, a gate electrode layer 920, a gate insulating film 930 inwhich an insulating film 931 and an insulating film 932 are stacked inthis order, an oxide semiconductor layer 940, and a source electrodelayer 950 and a drain electrode layer 960 in contact with part of theoxide semiconductor layer 940. In addition, an insulating film 980 andan insulating film 990 may be formed over the gate insulating film 930,the oxide semiconductor layer 940, the source electrode layer 950, andthe drain electrode layer 960.

The transistor of one embodiment of the present invention may include,as illustrated in FIG. 22B, a conductive film 921 that overlaps with thegate electrode layer 920 and the oxide semiconductor layer 940 and isover the insulating film 990. When the conductive film 921 is used as asecond gate electrode layer (back gate), the on-state current can beincreased and the threshold voltage can be controlled. To increase theon-state current, for example, the gate electrode layer 920 and theconductive film 921 are set to have the same potential, and thetransistor is driven as a dual-gate transistor. Further, to control thethreshold voltage, a fixed potential that is different from a potentialof the gate electrode layer 920 is supplied to the conductive film 921.

The transistor of one embodiment of the present invention may have achannel-protective bottom-gate structure as illustrated in FIGS. 23A and23B. In this structure, an insulating film 933 has a function ofprotecting a channel region. Thus, the insulating film 933 may beprovided only in a region overlapping with the channel region orprovided in not only the region but also a region other than the regionas illustrated in FIGS. 23A and 23B.

The transistor of one embodiment of the present invention may have aself-aligned top-gate structure as illustrated in FIGS. 24A and 24B. Inthe structure in FIG. 24A, a source region 951 and a drain region 961can be formed in the following manner: oxygen vacancies are generated bymaking the source electrode layer 950 and the drain electrode layer 960be in contact with the oxide semiconductor layer 940; and the oxidesemiconductor layer 940 is doped with impurities such as boron,phosphorus, or argon using the gate electrode layer 920 as a mask. Inthe structure in FIG. 24B, the source region 951 and the drain region961 can be formed, instead of using the doping method, in the followingmanner: an insulating film 975 containing hydrogen, such as a siliconnitride film, is formed to be in contact with part of the oxidesemiconductor layer 940 and the hydrogen is diffused to the part of theoxide semiconductor layer 940.

The transistor of one embodiment of the present invention may have aself-aligned top-gate structure as illustrated in FIG. 25A. In thestructure in FIG. 25A, the source region 951 and the drain region 961can be formed in the following manner: oxygen vacancies are generated bymaking the source electrode layer 950 and the drain electrode layer 960be in contact with the oxide semiconductor layer 940; and the oxidesemiconductor layer 940 is doped with impurities such as boron,phosphorus, or argon using the gate insulating film 930 as a mask. Inthe structure in FIG. 25A, the source electrode layer 950, the drainelectrode layer 960, and the gate electrode layer 920 can be formed inone process.

The transistor of one embodiment of the present invention may have aself-aligned top-gate structure as illustrated in FIG. 25B. In thestructure in FIG. 25B, the source region 951 and the drain region 961can be formed by, besides the doping with impurities such as boron,phosphorus, or argon using the gate insulating film 930 as a mask,diffusion of hydrogen from the insulating film 975 such as a siliconnitride film, which contains hydrogen and is in contact with part of theoxide semiconductor layer 940, to the part of the oxide semiconductorlayer 940. In the structure, the source region 951 and the drain region961 can have lower resistance. Alternatively, a structure in whichdoping with the impurities is not performed or a structure without theinsulating film 975 can be formed.

The transistor of one embodiment of the present invention may include aconductive film 921 overlapping with the oxide semiconductor layer 940with the insulating film 915 interposed therebetween as illustrated inFIGS. 26A and 26B. Although FIGS. 26A and 26B illustrate examples wherethe conductive film 921 is provided in the transistors illustrated inFIGS. 24A and 24B, the conductive film 921 can be provided in thetransistors illustrated in FIGS. 25A and 25B.

In the display device of one embodiment of the present invention, anoxide semiconductor can be used in an active layer as described above.The transistor using an oxide semiconductor layer has a higher mobilitythan a transistor using amorphous silicon, and is thus easily reduced insize, resulting in a reduction in the size of a pixel. The transistorusing an oxide semiconductor layer enables a flexible display device tohave high reliability. Note that one embodiment of the present inventionis not limited thereto. An active layer may include a semiconductorother than an oxide semiconductor depending on the case or condition.

Note that in the cross-sectional structures of the transistorsillustrated in FIGS. 22A and 22B and FIGS. 23A and 23B, the width of thegate electrode layer 920 is preferably larger than that of the oxidesemiconductor layer 940. In the display device having a backlight, thegate electrode layer functions as a light-blocking layer, and adeterioration of electrical characteristics, caused by irradiation ofthe oxide semiconductor layer 940 with light, can be suppressed. In adisplay device including an organic EL element or the like, a gateelectrode layer in a top-gate transistor can function as alight-blocking layer.

Next, the components of the transistor of one embodiment of the presentinvention will be described in detail.

The substrate 900 can be formed using a material that can be used forthe substrate 41 and the substrate 42 described in Embodiment 1. Notethat the substrate 900 corresponds to the substrate 41 in Embodiment 1.

As the insulating film 915, for example, a single layer of a siliconoxide film, a silicon oxynitride film, a silicon nitride film, or asilicon nitride oxide film, or a stacked layer including any of theabove films can be used. The insulating film 915 corresponds to theinsulating film 321 a in Embodiment 1.

The gate electrode layer 920 and the conductive film 921 can be formedusing a metal element selected from chromium (Cr), copper (Cu), aluminum(Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta),titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), andcobalt (Co), an alloy including the above metal element, an alloy inwhich any of the above metal elements are combined, or the like.Furthermore, the gate electrode layer 920 may have a single-layerstructure or a stacked structure of two or more layers.

Alternatively, the gate electrode layer 920 and the conductive film 921can be formed using a light-transmitting conductive material such asindium tin oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, indiumtin oxide to which silicon oxide is added, or graphene. A layeredstructure formed using the above light-transmitting conductive materialand the above metal element can also be employed.

Further, an In—Ga—Zn-based oxynitride semiconductor film, an In—Sn-basedoxynitride semiconductor film, an In—Ga-based oxynitride semiconductorfilm, an In—Zn-based oxynitride semiconductor film, a Sn-basedoxynitride semiconductor film, an In-based oxynitride semiconductorfilm, a film of metal nitride (such as InN or ZnN), or the like may beprovided between the gate electrode layer 920 and the insulating film932.

As each of the insulating films 931 and 932 functioning as the gateinsulating film 930, an insulating layer including at least one of thefollowing films formed by a plasma enhanced chemical vapor deposition(PECVD) method, a sputtering method, or the like can be used: a siliconoxide film, a silicon oxynitride film, a silicon nitride oxide film, asilicon nitride film, an aluminum oxide film, a hafnium oxide film, anyttrium oxide film, a zirconium oxide film, a gallium oxide film, atantalum oxide film, a magnesium oxide film, a lanthanum oxide film, acerium oxide film, and a neodymium oxide film. Note that instead of astacked structure of the insulating films 931 and 932, the gateinsulating film 930 may be an insulating film of a single layer formedusing a material selected from the above or an insulating film of threeor more layers.

Note that the insulating film 932 that is in contact with the oxidesemiconductor layer 940 functioning as a channel formation region of thetransistor is preferably an oxide insulating film and preferably has aregion (oxygen-excess region) containing oxygen in excess of thestoichiometric composition. In other words, the insulating film 932 isan insulating film from which oxygen can be released. In order toprovide the oxygen-excess region in the insulating film 932, theinsulating film 932 is formed in an oxygen atmosphere, for example.Alternatively, oxygen may be introduced into the deposited insulatingfilm 932 to provide the oxygen-excess region therein. Oxygen can beintroduced by an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, plasma treatment, or the like.

In the case where hafnium oxide is used for the insulating films 931 and932, the following effect is attained. Hafnium oxide has higherdielectric constant than silicon oxide and silicon oxynitride.Therefore, when hafnium oxide is used, a thickness can be made largerthan when silicon oxide is used; thus, leakage current due to tunnelcurrent can be low.

In this embodiment, a silicon nitride film is formed as the insulatingfilm 931, and a silicon oxide film is formed as the insulating film 932.A silicon nitride film has a higher dielectric constant than a siliconoxide film and needs a larger thickness for capacitance equivalent tothat of the silicon oxide. Thus, when a silicon nitride film is used forthe gate insulating film 930 of the transistor, the physical thicknessof the insulating film can be increased. From the above, theelectrostatic breakdown of the transistor can be prevented by improvingthe withstand voltage of the transistor.

The oxide semiconductor layer 940 is typically formed using an In—Gaoxide, an In—Zn oxide, or an In-M-Zn oxide (M is Ti, Ga, Y, Zr, La, Ce,Nd, Sn, or Hf). In particular, an In-M-Zn oxide (M is Ti, Ga, Y, Zr, La,Ce, Nd, Sn, or Hf) is preferably used for the oxide semiconductor layer940.

In the case where the oxide semiconductor layer 940 is an In-M-Zn oxide(M is Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf), it is preferable that theatomic ratio of metal elements of a sputtering target used for forming afilm of the In-M-Zn oxide satisfy In M and Zn≧M. The sputtering targetis preferably polycrystalline. As the atomic ratio of metal elements ofsuch a sputtering target, In:M:Zn=1:1:1, In:M:Zn=5:5:6, In:M:Zn=3:1:2,In:M:Zn=2:1:3, and the like are preferable. Note that the atomic ratioof metal elements in the formed oxide semiconductor layer 940 variesfrom the above atomic ratio of metal elements of the sputtering targetwithin a range of ±40% as an error.

In the case of using an In-M-Zn oxide for the oxide semiconductor layer940, when Zn and O are eliminated from consideration, the proportion ofIn and the proportion of M are greater than 25 atomic % and less than 75atomic %, respectively, preferably greater than 34 atomic % and lessthan 66 atomic %, respectively.

The energy gap of the oxide semiconductor layer 940 is 2 eV or more,preferably 2.5 eV or more, and more preferably 3 eV or more. In thismanner, the amount of off-state current of a transistor can be reducedby using an oxide semiconductor having a wide energy gap.

The oxide semiconductor layer 940 has a thickness greater than or equalto 3 nm and less than or equal to 200 nm, preferably 3 nm to 100 nm, andfurther preferably 3 nm to 50 nm.

An oxide semiconductor layer with low carrier density is used as theoxide semiconductor layer 940. For example, an oxide semiconductor layerwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, further preferably lower than orequal to 1×10¹³/cm³, still further preferably lower than or equal to1×10¹¹/cm³ is used as the oxide semiconductor layer 940.

However, the composition is not limited to those described above, and amaterial having the appropriate composition may be used depending onrequired semiconductor characteristics and electrical characteristics ofthe transistor (e.g., field-effect mobility and threshold voltage).Further, in order to obtain the required semiconductor characteristicsof the transistor, it is preferable that the carrier density, theimpurity concentration, the defect density, the atomic ratio of a metalelement to oxygen, the interatomic distance, the density, and the likeof the oxide semiconductor layer 940 be set to appropriate values.

Further, in the oxide semiconductor layer, hydrogen, nitrogen, carbon,silicon, and metal elements except for main components are impurities.For example, hydrogen and nitrogen form donor levels to increase thecarrier density. Silicon forms impurity states in the oxidesemiconductor layer. The impurity state becomes a trap, which mightdeteriorate the electrical characteristics of the transistor. It ispreferable to reduce the concentration of the impurities in the oxidesemiconductor layer and at interfaces with other layers.

Note that stable electrical characteristics can be effectively impartedto a transistor in which an oxide semiconductor layer serves as achannel by reducing the concentration of impurities in the oxidesemiconductor layer to make the oxide semiconductor layer intrinsic orsubstantially intrinsic. The term “substantially intrinsic” refers tothe state where an oxide semiconductor layer has a carrier density lowerthan 1×10¹⁷/cm³, preferably lower than 1×10¹⁵/cm³, further preferablylower than 1×10¹³/cm³.

In order to make the oxide semiconductor layer intrinsic orsubstantially intrinsic, in SIMS (secondary ion mass spectrometry), forexample, the concentration of silicon at a certain depth of the oxidesemiconductor layer or in a region of the oxide semiconductor layer islower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³,more preferably lower than 1×10¹⁸ atoms/cm³. Further, the concentrationof hydrogen at a certain depth of the oxide semiconductor layer or in aregion of the oxide semiconductor layer is lower than or equal to 2×10²⁰atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, furtherpreferably lower than or equal to 1×10¹⁹ atoms/cm³, still furtherpreferably lower than or equal to 5×10¹⁸ atoms/cm³. Further, theconcentration of nitrogen at a certain depth of the oxide semiconductorlayer or in a region of the oxide semiconductor layer is lower than5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³,further preferably lower than or equal to 1×10¹⁸ atoms/cm³, stillfurther preferably lower than or equal to 5×10¹⁷ atoms/cm³.

In the case where the oxide semiconductor layer includes crystals, highconcentration of silicon or carbon might reduce the crystallinity of theoxide semiconductor layer. In order not to lower the crystallinity ofthe oxide semiconductor layer, for example, the concentration of siliconat a certain depth of the oxide semiconductor layer or in a region ofthe oxide semiconductor layer is lower than 1×10¹⁹ atoms/cm³, preferablylower than 5×10¹⁸ atoms/cm³, further preferably lower than 1×10¹⁸atoms/cm³. Further, the concentration of carbon at a certain depth ofthe oxide semiconductor layer or in a region of the oxide semiconductorlayer is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸atoms/cm³, further preferably lower than 1×10¹⁸ atoms/cm³, for example.

Various experiments can prove low off-state current of a transistorincluding a highly purified oxide semiconductor layer for a channelformation region. For example, even when an element has a channel widthof 1×10⁶ μm and a channel length of 10 μm, off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at voltage (drainvoltage) between the source electrode and the drain electrode of from 1V to 10 V. In this case, it can be seen that the off-state currentnormalized on the channel width of the transistor is lower than or equalto 100 zA/μm. In addition, a capacitor and a transistor are connected toeach other and the off-state current is measured with a circuit in whichcharge flowing into or from the capacitor is controlled by thetransistor. In the measurement, a highly purified oxide semiconductorlayer is used for a channel formation region of the transistor, and theoff-state current of the transistor is measured by a change in theamount of electric charge of the capacitor per unit time. As a result,it is found that in the case where the voltage between the sourceelectrode and the drain electrode of the transistor is 3 V, loweroff-state current of several tens of yoctoamperes per micrometer (yA/μm)can be obtained. Accordingly, the off-state current of the transistorincluding a channel formation region formed of the highly purified oxidesemiconductor layer is considerably lower than that of a transistorincluding silicon having crystallinity.

For the source electrode layer 950 and the drain electrode layer 960, aconductive film having properties of extracting oxygen from the oxidesemiconductor layer is preferably used. For example, Al, Cr, Cu, Ta, Ti,Mo, W, Ni, Mn, Nd, or Sc can be used. An alloy or a conductive nitrideof any of these materials can also be used. A stack including aplurality of materials selected from these materials, alloys of thesematerials, and conductive nitrides of these materials can also be used.Typically, it is preferable to use Ti, which is particularly easilybonded to oxygen, or W, which has a high melting point and thus allowssubsequent process temperatures to be relatively high. Alternatively, Cuor a Cu—X alloy (X indicates Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti), whichhas low resistance may be used. Further alternatively, a stacked layerincluding any of the above materials and Cu or a Cu—X alloy may be used.

In the case of using a Cu—X alloy (X indicates Mn, Ni, Cr, Fe, Co, Mo,Ta, or Ti), a covering film is formed in a region in contact with theoxide semiconductor layer or a region in contact with an insulating filmby heat treatment, in some cases. The covering film includes a compoundcontaining X Examples of a compound containing X include an oxide of X,an In—X oxide, a Ga—X oxide, an In—Ga—X oxide, and In—Ga—Zn—X oxide.When the covering film is formed, the covering film functions as ablocking film, and Cu in the Cu—X alloy film can be prevented fromentering the oxide semiconductor layer.

By the conductive film capable of extracting oxygen from the oxidesemiconductor layer, oxygen in the oxide semiconductor layer is releasedto form oxygen vacancies in the oxide semiconductor layer. Hydrogenslightly contained in the layer and the oxygen vacancy are bonded toeach other, whereby the region is markedly changed to an n-type region.Accordingly, the n-type regions can serve as a source or a drain regionof the transistor.

The insulating films 980 and 990 each have a function of a protectiveinsulating film. The insulating film 980 is formed using an oxideinsulating film containing oxygen in excess of that in thestoichiometric composition. Part of oxygen is released by heating fromthe oxide insulating film containing oxygen in excess of that in thestoichiometric composition. The oxide insulating film containing oxygenin excess of that in the stoichiometric composition is an oxideinsulating film of which the amount of released oxygen converted intooxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, preferablygreater than or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis in whichheat treatment is performed such that a temperature of a film surface ishigher than or equal to 100° C. and lower than or equal to 700° C.,preferably higher than or equal to 100° C. and lower than or equal to500° C.

Silicon oxide, silicon oxynitride, or the like with a thickness greaterthan or equal to 30 nm and less than or equal to 500 nm, preferablygreater than or equal to 50 nm and less than or equal to 400 nm can beused as the insulating film 980.

Further, it is preferable that the amount of defects in the insulatingfilm 980 be small, typically the spin density of a signal which appearsat g=2.001 originating from a dangling bond of silicon, be lower than1.5×10¹⁸ spins/cm³, further preferably lower than or equal to 1×10¹⁸spins/cm³ by ESR measurement. Note that the insulating film 980 isprovided more apart from the oxide semiconductor layer 940 than theinsulating film 970 is; thus, the insulating film 980 may have higherdefect density than the insulating film 970.

The insulating film 990 has a function of blocking oxygen, hydrogen,water, an alkali metal, an alkaline earth metal, or the like. With theinsulating film 990, oxygen diffusion from the oxide semiconductor layer940 to the outside and entry of hydrogen, water, or the like from theoutside to the oxide semiconductor layer 940 can be prevented. As theinsulating film 990, a nitride insulating film can be used, for example.The nitride insulating film is formed using silicon nitride, siliconnitride oxide, aluminum nitride, aluminum nitride oxide, or the like.Note that instead of the nitride insulating film having a blockingeffect against oxygen, hydrogen, water, alkali metal, alkaline earthmetal, and the like, an oxide insulating film having a blocking effectagainst oxygen, hydrogen, water, and the like, may be provided. Theoxide insulating film having a blocking effect against oxygen, hydrogen,water, and the like is formed using aluminum oxide, aluminum oxynitride,gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride,hafnium oxide, hafnium oxynitride, or the like.

Note that the oxide semiconductor layer 940 may have a structure inwhich a plurality of oxide semiconductor layers are stacked. Forexample, as in a transistor illustrated in FIG. 27A, stacked layers of afirst oxide semiconductor layer 941 a and a second oxide semiconductorlayer 941 b may constitute the oxide semiconductor layer 940. The firstoxide semiconductor layer 941 a and the second oxide semiconductor layer941 b may include metal oxides having different atomic ratios. Forexample, one of the oxide semiconductor layers may include one of anoxide containing two kinds of metals, an oxide containing three kinds ofmetals, and an oxide containing four kinds of metals, and the other ofthe oxide semiconductor layers may include another one of the oxidecontaining two kinds of metals, the oxide containing three kinds ofmetals, and the oxide containing four kinds of metals.

Alternatively, the first oxide semiconductor layer 941 a and the secondoxide semiconductor layer 941 b may include the same constituentelements with different atomic ratios. For example, one of the oxidesemiconductor layers may contain In, Ga, and Zn at an atomic ratio of1:1:1, 5:5:6, 3:1:2, or 2:1:3 and the other of the oxide semiconductorlayers may contain In, Ga, and Zn at an atomic ratio of 1:3:2, 1:3:4,1:3:6, 1:4:5, 1:6:4, or 1:9:6. Note that the atomic ratio of each oxidesemiconductor layer varies within a range of ±40% of the above atomicratio as an error.

In the above, one of the oxide semiconductor layers, which is closer tothe gate electrode (the oxide semiconductor layer on the channel side),has an atomic ratio of In≧Ga (in the atomic ratio, In is greater than orequal to Ga); and the other oxide semiconductor layer, which is fartherfrom the gate electrode (the oxide semiconductor layer on the backchannel side), has an atomic ratio of In<Ga. In that case, a transistorwith a high field-effect mobility can be manufactured. On the otherhand, when the oxide semiconductor layer on the channel side has anatomic ratio of In<Ga and the oxide semiconductor layer on the backchannel side has an atomic ratio of In≧Ga (in the atomic ratio, In isgreater than or equal to Ga), the amount of change in the thresholdvoltage of a transistor due to change over time or a reliability testcan be reduced.

Further alternatively, the semiconductor film of the transistor may havea three-layer structure of a first oxide semiconductor layer, a secondoxide semiconductor layer, and a third oxide semiconductor layer. Inthat case, the first to third oxide semiconductor layers may include thesame constituent elements with different atomic ratios. A transistorincluding a three-layer semiconductor film will be described withreference to FIG. 27B and FIGS. 28A and 28B. Note that a structure inwhich a semiconductor film has a stacked structure can be employed forother transistors described in this embodiment.

Each of the transistors illustrated in FIG. 27B and FIGS. 28A and 28Bincludes a first oxide semiconductor layer 942 a, a second oxidesemiconductor layer 942 b, and a third oxide semiconductor layer 942 cwhich are stacked in this order from a gate insulating film side.

The first oxide semiconductor layer 942 a and the third oxidesemiconductor layer 942 c are formed using a material represented byInM_(1x)Zn_(y)O_(z) (x≧1 (x is greater than or equal to 1), y>1, z>0,M₁=Ga, Hf, or the like). The second oxide semiconductor layer 942 b isformed using a material which can be represented by InM_(2x)Zn_(y)O_(z)(x≧1 (x is greater than or equal to 1), y≧x (y is greater than or equalto x), z>0, M₂=Ga, Sn, or the like).

Materials of the first to third oxide semiconductor layers are selectedas appropriate so as to form a well-shaped structure in which theconduction band minimum in the second oxide semiconductor layer 942 b isdeeper from the vacuum level than the conduction band minimum in thefirst and third oxide semiconductor layers 942 a and 942 c.

For example, the first oxide semiconductor layer 942 a and the thirdoxide semiconductor layer 942 c may each have an atomic ratio ofIn:Ga:Zn=1:1:1, 1:3:2, 1:3:4, 1:3:6, 1:4:5, 1:6:4, or 1:9:6; the secondoxide semiconductor layer 942 b may have an atomic ratio ofIn:Ga:Zn=1:1:1, 5:5:6, 3:1:2, or 2:1:3.

Since the first to third oxide semiconductor layers 942 a to 942 cinclude the same constituent elements, the second oxide semiconductorlayer 942 b has few defect states (trap states) at the interface withthe first oxide semiconductor layer 942 a. Specifically, the defectstates (trap states) are fewer than those at the interface between thegate insulating film and the first oxide semiconductor layer 942 a. Forthis reason, when the oxide semiconductor layers are stacked in theabove manner, the amount of change in the threshold voltage of atransistor due to a change over time or a reliability test can bereduced.

Further, a well-shaped structure is preferably formed in which theconduction band minimum in the second oxide semiconductor layer 942 b isdeeper from the vacuum level than the conduction band minimum in thefirst and third oxide semiconductor layers 942 a and 942 c. Whenmaterials of the first to third oxide semiconductor layers are selectedas appropriate, the field-effect mobility of the transistor can beincreased and the amount of change in the threshold voltage of thetransistor due to change over time or a reliability test can be reduced.

Further, the first to third oxide semiconductor layers 942 a to 942 cmay be formed using oxide semiconductors having differentcrystallinities. Note that at least the second oxide semiconductor layer942 b that can function as a channel formation region is preferably afilm with crystallinity, further preferably a film in which c-axes arealigned in a direction substantially perpendicular to a surface.

The top-gate transistor illustrated in FIG. 28A or the like preferablyhas any of cross-sectional structures illustrated in FIG. 29A in thechannel width direction of a channel formation region. In each of theabove structures, the gate electrode layer 920 electrically surroundsthe oxide semiconductor layer 940 in the channel width direction. Thisstructure increases the on-state current. Such a transistor structure isreferred to as a surrounded channel (s-channel) structure.

In the structure including the conductive film 921 as illustrated inFIGS. 26A and 26B, the gate electrode layer 920 and the conductive film921 may be connected to each other through a contact hole, asillustrated in FIG. 29B, so as to have the same potential.

Furthermore, top-view structures of the source electrode layer 950 andthe drain electrode layer 960 of the transistor of one embodiment of thepresent invention can be as illustrated in FIGS. 30A and 30B. Note thatFIGS. 30A and 30B each illustrate only the oxide semiconductor layer940, the source electrode layer 950, and the drain electrode layer 960.As shown in FIG. 30A, the width (W_(SD)) of each of the source electrodelayer 950 and the drain electrode layer 960 may be larger than the width(W_(OS)) of the oxide semiconductor layer 940. Alternatively, W_(SD) maybe smaller than W_(OS) as shown in FIG. 30B. When W_(OS)≧W_(SD) (W_(SD)is less than or equal to W_(OS)) is satisfied, a gate electric field iseasily applied to the entire oxide semiconductor layer 940, so thatelectrical characteristics of the transistor can be improved.

Although the variety of films such as the metal films, the semiconductorfilms, and the inorganic insulating films which are described in thisembodiment typically can be formed by a sputtering method or a plasmaCVD method, such films may be formed by another method, e.g., a thermalCVD method. A metal organic chemical vapor deposition (MOCVD) method oran atomic layer deposition (ALD) method may be employed as an example ofa thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to a chamber at a time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is caused in the vicinity of thesubstrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching the respective switching valves (also referredto as high-speed valves) such that the source gases are not mixed. Forexample, a first source gas is introduced, an inert gas (e.g., argon ornitrogen) or the like is introduced at the same time as or after theintroduction of the first gas, and then a second source gas isintroduced. Note that in the case where the first source gas and theinert gas are introduced at a time, the inert gas serves as a carriergas, and the inert gas may also be introduced at the same time as theintroduction of the second source gas. Alternatively, the first sourcegas may be exhausted by vacuum evacuation instead of the introduction ofthe inert gas, and then the second source gas may be introduced. Thefirst source gas is adsorbed on the surface of the substrate to form afirst layer; then, the second source gas is introduced to react with thefirst layer; as a result, a second layer is stacked over the firstlayer, so that a thin film is formed. The sequence of the gasintroduction is controlled and repeated plural times until a desiredthickness is obtained, whereby a thin film with excellent step coveragecan be formed. The thickness of the thin film can be adjusted by thenumber of repetition times of the sequence of the gas introduction;therefore, an ALD method makes it possible to adjust a thicknessaccurately and thus is suitable for manufacturing a minute FET.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film which have been disclosed in theembodiments can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O film isformed, trimethylindium, trimethylgallium, and dimethylzinc can be used.Note that the chemical formula of trimethylindium is In(CH₃)₃. Thechemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formulaof dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, i.e.,ozone (O₃) as an oxidizer and a source gas which is obtained byvaporizing liquid containing a solvent and a hafnium precursor compound(hafnium alkoxide or hafnium amide such astetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemicalformula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples ofanother material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, e.g., H₂O as anoxidizer and a source gas which is obtained by vaporizing a solvent andliquid containing an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using ALD, hexachlorodisilane is adsorbed on asurface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times toform an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at atime to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas which is obtainedby bubbling with an inert gas such as Ar may be used instead of an O₃gas, it is preferable to use an O₃ gas, which does not contain H.Further, instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used.Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Further, aZn(CH₃)₂ gas may be used.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 4

A structure of an oxide semiconductor film which can be used for oneembodiment of the present invention is described below.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 80° and less thanor equal to 100°, and accordingly also includes the case where the angleis greater than or equal to 85° and less than or equal to 95°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

When observing the CAAC-OS film in a combined analysis image of abright-field image and a diffraction pattern with the use of atransmission electron microscope (TEM) (the combined analysis image isalso referred to as a high-resolution TEM image), a plurality of crystalparts can be found. However, in the high-resolution TEM image, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a morphology reflecting unevenness of asurface over which the CAAC-OS film is formed (hereinafter, a surfaceover which the CAAC-OS film is formed is referred to as a formationsurface) or a top surface of the CAAC-OS film, and is arranged parallelto the formation surface or the top surface of the CAAC-OS film.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface, metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 28 may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak at 2θ of around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancy in the oxide semiconductorfilm serves as a carrier trap or serves as a carrier generation sourcewhen hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, the transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film is described.

A microcrystalline oxide semiconductor film has a region where a crystalpart is observed in a high-resolution TEM image and a region where acrystal part is not clearly observed in a high-resolution TEM image. Inmost cases, a crystal part in the microcrystalline oxide semiconductorfilm is greater than or equal to 1 nm and less than or equal to 100 nm,or greater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In a high-resolution TEMimage of the nc-OS film, for example, a grain boundary is not clearlyobserved in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Note that there isno regularity of crystal orientation between different crystal parts inthe nc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak which showsa crystal plane does not appear. Furthermore, a diffraction pattern likea halo pattern appears in a selected-area electron diffraction patternof the nc-OS film which is obtained by using an electron beam having aprobe diameter (e.g., larger than or equal to 50 nm) larger than thediameter of a crystal part. Meanwhile, spots appear in a nanobeamelectron diffraction pattern of the nc-OS film obtained by using anelectron beam having a probe diameter close to, or smaller than thediameter of a crystal part. Further, in a nanobeam electron diffractionpattern of the nc-OS film, regions with high luminance in a circular(ring) pattern are shown in some cases. Also in a nanobeam electrondiffraction pattern of the nc-OS film, a plurality of spots are shown ina ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state as in quartz.

In the high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Further, a halo pattern is shown but a spot is not shown in a nanobeamelectron diffraction pattern of the amorphous oxide semiconductor film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(amorphous-like OS).

In a high-resolution TEM image of the amorphous-like OS film, a void isobserved in some cases. Furthermore, in the high-resolution TEM image,there are a region where a crystal part is clearly observed and a regionwhere a crystal part is not observed. In the amorphous-like OS film,crystallization occurs by a slight amount of electron beam used for TEMobservation and growth of the crystal part is found in some cases. Incontrast, crystallization by a slight amount of electron beam used forTEM observation is hardly observed in the nc-OS film having goodquality.

Note that the crystal part size in the amorphous-like OS film and thenc-OS film can be measured using high-resolution TEM images. Forexample, an InGaZnO₄ crystal has a layered structure in which twoGa—Zn—O layers are included between In—O layers. A unit cell of theInGaZnO₄ crystal has a structure in which nine layers of three In—Olayers and six Ga—Zn—O layers are layered in the c-axis direction.Accordingly, the spacing between these adjacent layers is equivalent tothe lattice spacing on the (009) plane (also referred to as d value).The value is calculated to 0.29 nm from crystal structure analysis.Thus, focusing on lattice fringes in the high-resolution TEM image, eachof lattice fringes in which the lattice spacing therebetween is greaterthan or equal to 0.28 nm and less than or equal to 0.30 nm correspondsto the a-b plane of the InGaZnO₄ crystal.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, anamorphous-like OS film, a microcrystalline oxide semiconductor film, anda CAAC-OS film, for example.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 5

In this embodiment, a display module which can be formed using a displaydevice of one embodiment of the present invention will be described.

In a display module 8000 in FIG. 31, a touch panel 8004 connected to anFPC 8003, a display panel 8006 connected to an FPC 8005, a backlightunit 8007, a frame 8009, a printed circuit board 8010, and a battery8011 are provided between an upper cover 8001 and a lower cover 8002.

The display device of one embodiment of the present invention can beused for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006. The upper cover 8001 and thelower cover 8002 have flexibility.

The touch panel 8004 is, typically, a resistive touch panel or acapacitive touch panel and overlaps with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 so that the touch panel 8004 can function as anoptical touch panel. The touch panel 8004 may have flexibility. Thefunction of the touch panel may be achieved by the use of the distortionsensor element of one embodiment of the present invention.

The backlight unit 8007 includes a light source 8008. Note that althougha structure in which the light sources 8008 are provided over thebacklight unit 8007 is illustrated in FIG. 31, one embodiment of thepresent invention is not limited to this structure. For example, astructure in which a light source 8008 is provided at an end of thebacklight unit 8007 and a light diffusion plate is further provided maybe employed. In the case where a self-luminous light-emitting elementsuch as an organic EL element is used or the case where a reflectivepanel is used, the backlight unit 8007 is not necessarily provided. Thebacklight unit 8007 may have flexibility.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed circuit board 8010. The frame 8009 canfunction as a radiator plate. The frame 8009 may have flexibility.

The printed circuit board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or a power source using the battery8011 provided separately may be used. The battery 8011 can be omitted inthe case of using a commercial power source. The printed circuit board8010 may be an FPC.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 6

In this embodiment, a touch panel that can include a display deviceprovided with the distortion sensor of one embodiment of the presentinvention will be described.

The display device of one embodiment of the present invention can have astructure in which a touch sensor, a display element, and a distortionsensor element are provided between a pair of substrates. The touchsensor is a capacitive type touch sensor, for example. The touch panelallows not only input by local touch on a display part but also input bybending of the display part owing to the distortion sensor element. Notethat the description of the distortion sensor element in the aboveembodiment is not made below.

In a touch panel including a touch sensor part and a display part thatoverlap with each other, a parasitic capacitance is formed in some casesbetween a wiring or an electrode included in a capacitive type touchsensor and a wiring or an electrode included in the display part. Noisecaused by operating the display element travels to the touch sensor sidethrough the parasitic capacitance and the detection sensitivity of thetouch sensor might decrease.

By sufficiently increasing the distance between the touch sensor partand the display part, the adverse effect of the noise can be avoided andthe decrease in the detection sensitivity of the touch sensor can besuppressed; however, the thickness of the whole touch panel is increasedin some cases.

This embodiment describes an example of using an active matrix touchsensor. The touch sensor includes a transistor and a capacitor. Thetransistor and the capacitor are electrically connected to each other.Note that a touch sensor without an element such as a transistor may beused for the display device of one embodiment of the present invention.

In the active matrix touch sensor of one embodiment of the presentinvention, an electrode of a capacitor and a read wiring can be formedin different layers. When the read wiring has a narrow width, aparasitic capacitance can be small and the adverse effect of the noisecan be suppressed. Accordingly, a decrease in the detection sensitivityof the touch sensor can be suppressed. In addition, by amplifying adetection signal and outputting the amplified signal, the adverse effectof the noise can be suppressed.

An active matrix touch sensor is used in the touch panel of oneembodiment of the present invention, whereby the distance between thesensor part and the display part can be reduced in the touch panel, andthe touch panel can have a small thickness. Furthermore, the touchsensor and the display element can be located between two substrates,whereby the touch panel can have a small thickness. Here, using thetouch sensor of one embodiment of the present invention can suppress adecrease in the detection sensitivity of the touch sensor even when thedistance between the sensor part and the display part is reduced.Therefore, in one embodiment of the present invention, both a smallthickness and high detection sensitivity of a touch sensor or a touchpanel can be achieved. Furthermore, by using a flexible material for thepair of substrates of the touch panel, the touch panel can haveflexibility. Furthermore, in one embodiment of the present invention, atouch panel with high resistance to repeated bending can be provided.Furthermore, a large-sized touch panel can be provided.

The touch sensor included in the touch panel of one embodiment of thepresent invention may include an oxide conductor layer as the electrodeof the capacitor. In the active matrix touch sensor, a semiconductorlayer and a conductive film of the transistor and the electrode of thecapacitor are preferably formed in the same step. Thus, the number ofsteps of manufacturing the touch panel can be reduced and the cost ofmanufacturing the touch panel can be reduced.

Note that because the oxide conductor layer is used as the electrode ofthe capacitor in the touch panel of one embodiment of the presentinvention, viewing angle dependence is smaller than that of a touchpanel using another material as the electrode of the capacitor in somecases. Furthermore, because the oxide conductor layer is used as theelectrode of the capacitor in the touch panel of one embodiment of thepresent invention, an NTSC ratio can be higher than that of a touchpanel using another material as the electrode of the capacitor in somecases.

Specifically, one embodiment of the present invention is a touch panelincluding a touch sensor, a light-blocking layer, and a display element.In the touch panel, the light-blocking layer is located between thetouch sensor and the display element, the light-blocking layer includesa region overlapping with a transistor included in the touch sensor, andthe display element includes a region overlapping with a capacitorincluded in the touch sensor. Note that the touch panel allows datainput without contact.

The display element can be, but is not particularly limited to, anorganic EL element.

<Structure Example of Touch Panel>

FIGS. 32A and 32B are projection views illustrating components of atouch panel of one embodiment of the present invention. FIG. 32A is aprojection view of a touch panel 700 of one embodiment of the presentinvention and a sensor unit 800 included in the touch panel 700.

The touch panel 700 described in this embodiment includes a flexibleinput device 100 and a display part 701 (see FIGS. 32A and 32B). Theflexible input device 100 is provided with a plurality of sensor units800 arranged in matrix and including windows 14 that transmit visiblelight; a scan line G1 electrically connected to the plurality of sensorunits 800 arranged in the row direction (shown by an arrow R in FIG.32A); a signal line DL electrically connected to the plurality of sensorunits 800 arranged in the column direction (shown by an arrow C in FIG.32A); and a flexible base material 16 supporting the sensor units 800,the scan line G1, and the signal line DL. The display part 701 isprovided with a plurality of pixels 702 overlapping with the windows 14and arranged in matrix; and a flexible base material 710 supporting thepixels 702.

The sensor unit 800 includes a sensor element C overlapping with thewindows 14 and a sensing circuit 19 electrically connected to the sensorelement C (FIG. 32A).

The sensing circuit 19 is supplied with a selection signal, and suppliesa sensor signal DATA in accordance with a change in capacitance of thesensor element C.

The scan line G1 can supply a selection signal. The signal line DL cansupply the sensor signal DATA. The sensing circuit 19 is provided so asto overlap with a gap between the windows 14.

In addition, the touch panel 700 described in this embodiment includes acoloring layer between the sensor unit 800 and the pixel 702 overlappingwith the window 14 of the sensor unit 800.

The touch panel 700 described in this embodiment includes the flexibleinput device 100 including the plurality of sensor units 800, each ofwhich is provided with the windows 14 transmitting visible light, andthe flexible display part 701 including the plurality of pixels 702overlapping with the windows 14. The coloring layer is included betweenthe window 14 and the pixel 702.

With such a structure, the touch panel can supply a sensor signal basedon the change in the capacitance and positional data of the sensor unitsupplying the sensor signal, can display image data associated with thepositional data of the sensor unit, and can be bent. As a result, anovel touch panel with high convenience or high reliability can beprovided.

The touch panel 700 may include an FPC 712 to which a signal from theinput device 100 is supplied and/or an FPC 713 supplying a signalincluding image data to the display part 701.

In addition, a protective layer 17 p protecting the touch panel 700 bypreventing damage and/or an anti-reflective layer 767 p that weakens theintensity of external light reflected by the touch panel 700 may beincluded.

Moreover, the touch panel 700 includes a scan line driver circuit 703 gwhich supplies the selection signal to a scan line of the display part701, a wiring 711 supplying a signal, and a terminal 719 electricallyconnected to the FPC 713.

Components of the touch panel 700 are described below. Note that thesecomponents cannot be clearly distinguished and one component also servesas another component or includes part of another component in somecases.

For example, the input device 100 including the coloring layeroverlapping with the plurality of windows 14 also serves as a colorfilter.

Furthermore, for example, the touch panel 700 in which the input device100 overlaps the display part 701 serves as the input device 100 as wellas the display part 701.

The touch panel 700 includes the input device 100 and the display part701 (FIG. 32A).

The input device 100 includes the plurality of sensor units 800 and theflexible base material 16 supporting the sensor units. For example, theplurality of sensor units 800 are arranged in matrix with 40 rows and 15columns on the flexible base material 16.

The window 14 transmits visible light.

For example, the window 14 may be formed as follows: the base material16, the sensor element C, and a flexible protective base material 17each formed using a material transmitting visible light or a materialthin enough to transmit visible light overlap with each other such thattransmission of visible light is not prevented.

For example, an opening may be provided in a material that does nottransmit visible light. Specifically, one opening or a plurality ofopenings having any of a variety of shapes such as a rectangle may beprovided.

A coloring layer that transmits light of a predetermined color isprovided to overlap with the window 14. For example, a coloring layerCFB transmitting blue light, a coloring layer CFG transmitting greenlight, and a coloring layer CFR transmitting red light are included(FIG. 32A).

Note that in addition to the coloring layers transmitting blue light,green light, and/or red light, coloring layers transmitting light ofvarious colors such as a coloring layer transmitting white light and acoloring layer transmitting yellow light can be included.

For a coloring layer, a metal material, a resin material, a pigment,dye, or the like can be used.

A light-blocking layer BM is provided to surround the windows 14. Thelight-blocking layer BM does not easily transmit light as compared tothe window 14.

For the light-blocking layer BM, carbon black, a metal oxide, acomposite oxide containing a solid solution of a plurality of metaloxides, or the like can be used.

The scan line G1, the signal line DL, a wiring VPI, a wiring RES, awiring VRES, and the sensing circuit 19 are provided to overlap with thelight-blocking layer BM.

Note that a light-transmitting overcoat covering the coloring layer andthe light-blocking layer BM can be provided.

As the flexible base material 16 and the flexible base material 710, anorganic material, an inorganic material, or a composite material of anorganic material and an inorganic material can be used.

For the base material 16 and the base material 710, a material with athickness of 5 μm or more and 2500 μm or less, preferably 5 μm or moreand 680 μm or less, further preferably 5 μm or more and 170 μm or less,still further preferably 5 μm or more and 45 μm or less, yet stillfurther preferably 8 μm or more and 25 μm or less can be used.

A material with which passage of impurities is inhibited can befavorably used for the base material 16 and the base material 710. Forexample, a material with a vapor permeability of lower than or equal to10⁻⁵ g/(m²·day), preferably lower than or equal to 10⁻⁶ g/(m²·day) canbe favorably used.

The base material 710 can be favorably formed using a material whosecoefficient of linear expansion is substantially equal to that of thematerial of the base material 16. For example, the base material 710 andthe base material 16 can each be formed using a material whosecoefficient of linear expansion is preferably lower than or equal to1×10⁻³/K, further preferably lower than or equal to 5×10⁻⁵/K, and stillfurther preferably lower than or equal to 1×10⁻⁵/K.

Examples of the materials of the base material 16 and the base material710 are organic materials such as a resin, a resin film, and a plasticfilm.

Examples of the materials of the base material 16 and the base material710 are inorganic materials such as a metal plate and a thin glass platewith a thickness of more than or equal to 10 μm and less than or equalto 50 μm.

The input device 100 can include the flexible protective base material17 and/or the protective layer 17 p. The flexible protective basematerial 17 or the protective layer 17 p protects the input device 100by preventing damage.

The display part 701 includes a plurality of pixels 702 arranged inmatrix (FIG. 32B). For example, the pixel 702 includes a sub-pixel 702B,a sub-pixel 702G, and a sub-pixel 702R, and each sub-pixel includes adisplay element and a pixel circuit for driving the display element.

Note that in the pixel 702, the sub-pixel 702B is located to overlapwith the coloring layer CFB, the sub-pixel 702G is located to overlapwith the coloring layer CFG, and the sub-pixel 702R is located tooverlap with the coloring layer CFR.

The display part 701 may include the anti-reflective layer 767 ppositioned in a region overlapping with pixels. As the anti-reflectivelayer 767 p, a circular polarizing plate can be used, for example.

The display part 701 includes the wiring 711 through which a signal canbe supplied. The wiring 711 is provided with the terminal 719. Note thatthe FPC 713 through which a signal such as an image signal or asynchronization signal can be supplied is electrically connected to theterminal 719.

Note that a printed wiring board (PWB) may be attached to the FPC 713.

<<Sensor Element C>>

The sensor element C is described giving an example that uses acapacitor. The capacitor includes a pair of electrodes. The capacitorincludes an insulating film as the dielectric layer between the pair ofelectrodes.

When an object whose dielectric constant is different from that of theair gets closer to one of the pair of electrodes of the sensor element Cthat is put in the air, the capacitance of the sensor element C ischanged. Specifically, when a finger or the like gets closer to thesensor element C, the capacitance of the sensor element C is changed.Thus, the sensor element C can be used in a proximity sensor.

When the sensor element C is changed in shape, the capacitance ischanged with the change in shape.

Specifically, when a finger or the like is in contact with the sensorelement C, and the gap between the pair of electrodes becomes small, thecapacitance of the sensor element C is increased. Accordingly, thesensor element C can be used in a tactile sensor.

Specifically, when the sensor element C is bent, and the gap between thepair of electrodes becomes small, the capacitance of the sensor elementC is increased. Accordingly, the sensor element C can also be used in abend sensor.

<<Sensing Circuit 19 and Converter CONV>>

FIGS. 33A, 33B1, 33B2, and 33C illustrate configurations of the sensingcircuit 19 and a converter CONV and a driving method of the sensingcircuit 19 that are embodiments of the present invention.

FIG. 33A is a circuit diagram illustrating configurations of the sensingcircuit 19 and the converter CONV of embodiments of the presentinvention, and FIGS. 33B1 and 33B2 are timing charts illustrating adriving method of one embodiment of the present invention. FIG. 33Cshows the converter CONV that is different from the converter CONV shownin FIG. 33A. FIG. 34A shows the sensor circuits 19 in matrix.

The sensing circuit 19 includes transistors M1 to M3, for example (FIG.33A and FIG. 34A). In addition, the sensing circuit 19 includes wiringsthat supply a power supply potential and a signal. For example, thesignal line DL, the wiring VPI, a wiring CS, the scan line G1, thewiring RES, the wiring VRES, and the like are included.

Note that the sensing circuit 19 may be located not to overlap with thewindow 14.

Furthermore, the transistors M1 to M3 each include a semiconductorlayer. For example, for the semiconductor layer, an element belonging toGroup 14, a compound semiconductor, or an oxide semiconductor can beused. Specifically, a semiconductor containing silicon, a semiconductorcontaining gallium arsenide, an oxide semiconductor containing indium,or the like can be used.

Transistors that can be formed in the same process can be used as thetransistors M1 to M3.

Any one of the transistors M1 to M3 preferably includes an oxidesemiconductor layer. At this time, the oxide semiconductor layer and theoxide conductor layer are preferably located over the same surface. Theoff-state current of a transistor including an oxide semiconductor layeris small; therefore, it is particularly preferable that the transistorM1 include the oxide semiconductor layer.

For the wiring, a conductive material can be used. For example, aninorganic conductive material, an organic conductive material, a metalmaterial, a conductive ceramic material, or the like can be used for thewiring. Specifically, a material that is the same as those of the pairof electrodes of the capacitor can be used.

For the scan line G1, the signal line DL, the wiring VPI, the wiringRES, and the wiring VRES, a metal material such as aluminum, gold,platinum, silver, nickel, titanium, tungsten, chromium, molybdenum,iron, cobalt, copper, or palladium, or an alloy material containing anyof these metal materials can be used.

The sensing circuit 19 may be formed over the base material 16 byprocessing a film formed over the base material 16.

Alternatively, the sensing circuit 19 formed over another base materialmay be transferred to the base material 16.

Various circuits that can convert the sensor signal DATA supplied fromthe sensor unit 800 and supply the converted signal to the FPC1 can beused as the converter CONV (FIG. 32A). For example, a transistor M4 canbe used in the converter CONV. Furthermore, as shown in FIG. 33C, thetransistor M4 and a transistor M5 can be used in the converter CONV.

The sensing circuit 19 of one embodiment of the present inventionincludes the transistor M1 whose gate is electrically connected to oneelectrode of the sensor element C and whose first electrode iselectrically connected to the wiring VPI that can supply a groundpotential, for example (see FIG. 33A).

The sensing circuit 19 may further include the transistor M2 whose gateis electrically connected to the scan line G1 that can supply aselection signal, whose first electrode is electrically connected to asecond electrode of the transistor M1, and whose second electrode iselectrically connected to the signal line DL that can supply the sensorsignal DATA, for example.

The sensing circuit 19 may further include the transistor M3 whose gateis electrically connected to the wiring RES that can supply a resetsignal, whose first electrode is electrically connected to the oneelectrode of the sensor element C, and whose second electrode iselectrically connected to the wiring VRES that can supply, for example,a ground potential.

The capacitance of the sensor element C is changed when an object getsclose to the pair of electrodes or when the distance between the pair ofelectrodes is changed. Thus, the sensing circuit 19 can supply thesensor signal DATA based on a change in the capacitance of the sensorelement C.

The sensing circuit 19 is provided with the wiring CS that can supply acontrol signal for controlling the potential of the other electrode ofthe sensor element C.

Note that a node at which the one electrode of the sensor element C, thegate of the transistor M1, and the first electrode of the transistor M3are electrically connected to one another is referred to as a node A.

The wiring VRES and wiring VPI each can supply a ground potential, forexample, and a wiring VPO and a wiring BR each can supply a high powersupply potential, for example. Furthermore, the wiring RES can supplythe reset signal, and the scan line G1 can supply the selection signal.Furthermore, the signal line DL can supply the sensor signal DATA, and aterminal OUT can supply a signal converted based on the sensor signalDATA.

Any of various circuits that can convert the sensor signal DATA andsupply the converted signal to the terminal OUT can be used as theconverter CONY. For example, a source follower circuit, a current mirrorcircuit, or the like may be formed by the electrical connection betweenthe converter CONV and the sensing circuit 19.

Specifically, by using the converter CONV including the transistor M4, asource follower circuit can be formed (FIG. 33A). Furthermore, as shownin FIG. 33C, the converter CONV may include the transistors M4 and M5.Note that transistors that can be formed in the same process as those ofthe transistor M1 to the transistor M3 may be used as the transistors M4and M5.

As described above, in the active matrix touch sensor of one embodimentof the present invention, the electrode of the sensor element and theread wiring can be formed in different layers. As shown in FIG. 34B, oneelectrode CM of a capacitor and a wiring ML are formed in differentlayers, and the width of the wiring ML is made narrow. Thus, theparasitic capacitance can be small and the adverse effect of noise canbe suppressed. Accordingly, a decrease in the detection sensitivity ofthe touch sensor can be suppressed. Note that the one electrode CM ofthe capacitor overlaps with the plurality of pixels 702 shown in FIG.34C that is an enlarged view of FIG. 34B.

<Driving Method of Sensing Circuit 19>

A driving method of the sensing circuit 19 is described.

<<First Step>>

In a first step, after the transistor M3 is turned on, a reset signalfor turning off the transistor M3 is supplied to the gate of thetransistor M3, so that the potential of the first electrode of thesensor element C is set to a predetermined potential (see Period T1 inFIG. 33B1).

Specifically, the reset signal is supplied from the wiring RES. Thetransistor M3 supplied with the reset signal renders the potential ofthe node A a ground potential, for example (see FIG. 33A).

<<Second Step>>

In a second step, a selection signal that turns on the transistor M2 issupplied to the gate of the transistor M2, and the second electrode ofthe transistor M1 is electrically connected to the signal line DL.

Specifically, the selection signal is supplied from the scan line G1.Through the transistor M2 to which the selection signal is supplied, thesecond electrode of the transistor M1 is electrically connected to thesignal line DL (see a period T2 in FIG. 33B1).

<<Third Step>>

In a third step, a control signal is supplied to the second electrode ofthe sensor element, and a potential changed in accordance with thecontrol signal and the capacitance of the sensor element C is suppliedto the gate of the transistor M1.

Specifically, a rectangular control signal is supplied from the wiringCS. The sensor element C whose second electrode is supplied with therectangular control signal increases the potential of the node A inaccordance with the capacitance of the sensor element C (see the latterpart of Period T2 in FIG. 33B1).

For example, in the case where the sensor element C is put in the air,when an object whose dielectric constant is higher than that of the airis placed closer to the second electrode of the sensor element C, thecapacitance of the sensor element C is apparently increased.

Thus, the change in the potential of the node A caused by therectangular control signal becomes smaller than that in the case wherean object whose dielectric constant is higher than that of the air isnot placed close to the second electrode of the sensor element C (see asolid line in FIG. 33B2).

<<Fourth Step>>

In a fourth step, a signal obtained by the change in the potential ofthe gate of the transistor M1 is supplied to the signal line DL.

For example, a change in current due to the change in the potential ofthe gate of the transistor M1 is supplied to the signal line DL.

The converter CONV converts the change in the current flowing throughthe signal line DL into a change in voltage and supplies the voltage.

<<Fifth Step>>

In a fifth step, a selection signal for turning off the transistor M2 issupplied to the gate of the transistor M2.

FIG. 35 is a cross-sectional view illustrating the structure of theabove-described touch panel. FIG. 35 illustrates an example in which thedisplay device in FIGS. 7A and 7B is provided with a touch sensor. Inthe touch panel, the transistor of the sensing circuit 19 and acapacitor 340 are formed on the second substrate 42 side.

One electrode 341 of the capacitor 340 is formed to overlap with thecoloring layer 336, using a material transmitting the light passingthrough the coloring layer 336. The other electrode 342 of the capacitor340 can be formed using, for example, the same material as asemiconductor layer of the transistor. For example, an oxide conductorlayer formed by adding impurities forming oxygen vacancies, impuritiesforming donor levels, or the like to an oxide semiconductor layer can beused.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 7

In this embodiment, electronic devices to which one embodiment of thepresent invention is applied will be described with reference to FIGS.36A and 36B.

Highly reliable flexible electronic devices can be manufactured byadopting the display device of one embodiment of the present invention.

Examples of the electronic devices to which the display device of oneembodiment of the present invention is applicable are a televisiondevice, a monitor of a computer or the like, digital signage, a digitalphoto frame, a mobile phone, a portable game console, a portableinformation terminal, an audio reproducing device, and the like.

FIG. 36A illustrates an example of a thin portable information terminal.A portable information terminal 7100 includes a display part 7102incorporated in a housing 7101, a speaker 7103, a microphone 7106, acamera 7107, and the like. Note that the housing 7101, the display part7102, and the like are flexible as shown in FIG. 36B, which leads toexcellent portability and high resistance against shock such as a dropimpact. The display part 7102 includes the display device of oneembodiment of the present invention, and for example, an image displayedon the display part can be changed by the action of bending or the like.Therefore, the display part 7102 is particularly suitable for anelectronic book or the like.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

This application is based on Japanese Patent Application serial no.2014-095200 filed with Japan Patent Office on May 2, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a first substrateand a second substrate each having flexibility; a first element layerbetween the first substrate and the second substrate, the first elementlayer including a display part and a first sensor element partoverlapping with the display part, wherein a top surface shape of thedisplay part is substantially rectangular; and a second element layerbetween the first substrate and the second substrate, the second elementlayer including second sensor element parts, wherein the first elementlayer and the second element layer overlap with each other, wherein thesecond sensor element parts overlap with a first region along a firstside of the display part, a second region along a second side facing thefirst side of the display part, and a third region located intermediatebetween the first side and the second side of the display part, whereinthe first sensor element part is configured to detect presence orabsence of an object touching the first substrate or the secondsubstrate, and wherein the second sensor element parts are configured todetect distortion of at least one of the first substrate and the secondsubstrate.
 2. The display device according to claim 1, furthercomprising an insulating layer between the first element layer and thesecond element layer.
 3. The display device according to claim 1,wherein the first sensor element part includes a metal thin filmresistor.
 4. The display device according to claim 1, wherein thedisplay part includes an organic EL element.
 5. A display modulecomprising: the display device according to claim 1; and at least one ofan FPC and a frame.
 6. An electronic appliance comprising: the displaydevice according to claim 1; and at least one of a speaker and a camera.7. A display device comprising: a first substrate and a second substrateeach having flexibility; a third substrate and a fourth substrate eachbeing provided between the first substrate and the second substrate,wherein the third substrate and the fourth substrate are provided nextto each other and do not overlap with each other; a first element layerbetween the first substrate and the second substrate and overlappingwith the third substrate and the fourth substrate, the first elementlayer including a display part and a first sensor element partoverlapping with the display part, wherein a top surface shape of thedisplay part is substantially rectangular; and a second element layerbetween the first substrate and the second substrate and overlappingwith the third substrate and the fourth substrate, the second elementlayer including second sensor element parts, wherein the first elementlayer and the second element layer overlap with each other, wherein thesecond sensor element parts overlap with a first region along a firstside of the display part, a second region along a second side facing thefirst side of the display part, and a third region located intermediatebetween the first side and the second side of the display part, whereinthe second sensor element parts overlap with the third substrate and thefourth substrate, wherein the first sensor element part is configured todetect presence or absence of an object touching the first substrate orthe second substrate, and wherein the second sensor element parts areconfigured to detect distortion of the first substrate or the secondsubstrate.
 8. The display device according to claim 7, wherein thedisplay part is capable of being bent along a region between the thirdsubstrate and the fourth substrate.
 9. The display device according toclaim 7, wherein each of the third substrate and the fourth substrate ismore rigid than the first substrate and the second substrate.
 10. Thedisplay device according to claim 7, wherein the third substrate and thefourth substrate have flexibility.
 11. The display device according toclaim 7, further comprising an insulating layer between the firstelement layer and the second element layer.
 12. The display deviceaccording to claim 7, wherein the first sensor element part includes ametal thin film resistor.
 13. The display device according to claim 7,wherein the display part includes an organic EL element.
 14. A displaymodule comprising: the display device according to claim 7; and at leastone of an FPC and a frame.
 15. An electronic appliance comprising: thedisplay device according to claim 7; and at least one of a speaker and acamera.
 16. A method for operating a display device, comprising thesteps of: displaying a first image on a display part; first determiningwhether a curvature of an edge or a vicinity of a center region of thedisplay part is larger than or equal to a first set value, wherein, whenthe curvature is smaller than the first set value, the step of firstdetermining of the curvature is performed again; displaying an image foroperation on the edge of the display part when the curvature is largerthan or equal to the first set value; first detecting, on the image foroperation, a position of an object touching the image; second detecting,on the image for operation, a change in the position of the object;second determining whether a curvature of the edge of the display partis larger than or equal to a second set value when the change in theposition of the object is not detected in the step of second detecting,wherein when the curvature is smaller than the second set value, thestep of second detecting is performed again; and changing the firstimage to a second image when the change in the position of the object isdetected in the step of second detecting, wherein, when the curvature islarger than or equal to the second set value in the step of seconddetermining, the step of changing is performed.
 17. The method foroperating a display device, according to claim 16, wherein the stepsfrom displaying to changing are performed sequentially and repeatedlywhile the second image displayed on the display part in the step ofchanging is used as the first image displayed on the display part in thestep of displaying.